J Clin Pathol: first published as 10.1136/jcp.28.Suppl_9.94 on 1 January 1975. Downloaded from

J. clin. Path., 28, Suppl. (Roy. Coll. Path.), 9, 94-98

The drug dilemma-benefits and hazards Drug interactions and lethal drug combinations

ALAN RICHENS From the Department of Clinical Pharmacology, St Bartholomew's Hospital, London

Although the development of drugs of greater example, aspirin can displace oral anticoagulants potency and efficacy confers on the physician from their plasma-protein-binding sites, and in- increasing power to treat serious diseases, it also directly acting sympathomimetics contained in cough increases the number and seriousness of potential mixtures can cause a hypertensive crisis in patients adverse effects and drug interactions which can on monoamine oxidase inhibitors. Self-medication is occur. Most hospital patients receive more than one common, and often involves drugs obtained on drug at a time, the average number often being prescription for a previous illness. greater than five (Smith, Seidl, and Cluff, 1966). The 4 When several clinics or doctors are involved in incidence of drug reactions rises with the number of the care of a patient, one doctor may not be aware of drugs prescribed simultaneously. In patients pre- what another has prescribed. scribed one to five drugs the incidence of reactions is 5 When preparations which contain more than 18-6%, while in patients prescribed six or more one ingredient are prescribed by their trade names. it rises to 814 % (Hurwitz and Wade, 1969). There are a number of ways in which drugs may The Boston Collaborative Drug Surveillance interact. These will be outlined and illustrated in the Program (1972) surveyed the incidence of drug remainder of this paper. reactions in 9900 patients admitted to nine acute copyright. disease hospitals and one chronic disease hospital. Interactions before Administration There were 83 200 drugs administered, and 3600 adverse reactions occurred of which 6-9 % were due Some drugs are chemically or physically incom- to drug interactions. The most serious interactions patible in the forms in which they are presented for included depression of the central nervous system, administration. For example, thiopentone sodium severe hypotension, gastrointestinal bleeding, psy- injection is strongly alkaline and will cause hydrolysis chotic behaviour and superinfection. Although the of suxamethonium if both drugs are drawn up into clinical importance of drug interactions has been a syringe together for combined anaesthetic induc- exaggerated in some reports, the number which end tion and muscle relaxation. The excess of protamine http://jcp.bmj.com/ in a fatal outcome is disproportionately high. In a zinc in protamine zinc insulin will combine with survey of 6199 medical inpatients in five general soluble insulin if the two preparations are mixed in hospitals and one chronic disease hospital, 744 the syringe before injection, resulting in a change in patients died in hospital, and of these deaths 27 were the rate of absorption of the latter preparation. As a considered to be due to drug treatment, of which nine general rule drug solutions should not be mixed, nor were caused by drug interactions (Shapiro, Slone, added to intravenous infusions, particularly whole Lewis, and Jick, 1971). blood, unless it is known that the components are on September 29, 2021 by guest. Protected The physician should be alert to the danger of chemically and physically compatible. drug interactions in a number of circumstances. Interactions in the Gut 1 When the pharmacological effect of a drug is harmful in excess and when the therapeutic ratio is Drugs are absorbed largely in the upper part of the low. Examples are hypoglycaemic drugs, anti- small intestine because of the enormous mucosal coagulants, digoxin, depressants of the central surface area and blood supply present there. Drugs nervous system, cytotoxic drugs. which alter the rate of gastric emptying can affect the 2 When a drug produces an altered state of delivery of the drug to this site and can thereby receptor sensitivity in the sympathetic nerve ter- influence either the rate or the extent of absorption. minals, eg, monoamine oxidase inhibitors (MA0Is), The anticholinergic properties of propantheline tricyclic antidepressants, hypotensive drugs. reduce the rate of gastric emptying and therefore 3 When interactions occur with medicines that slow down the absorption ofa drug such as paraceta- can be bought over the chemist's counter. For mol (Prescott, 1974). However, the extent of absorp- 94 J Clin Pathol: first published as 10.1136/jcp.28.Suppl_9.94 on 1 January 1975. Downloaded from

Drug interactions and lethal drug combinations 95 tion, particularly of poorly soluble drugs like metabolism is substantially reduced by inhibition of digoxin, may be increased. As many drugs possess the metabolic enzyme. It is probable that this partly anticholinergic effects, eg, antihistamines, tricyclic accounts for the hypertensive crisis which can result antidepressants, phenothiazines and anti-Parkin- from the administration of directly acting sympa- sonian drugs, this type of interaction probably thomimetic amines, eg, tyramine, ephedrine, levo- occurs commonly. Metoclopramide increases the dopa, to patients on chronic MAOI therapy. An rate of gastric emptying, particularly in patients in interaction at the sympathetic nerve terminal is also whom it is delayed, and has been shown to shorten important in causing the crisis (see below). the time to peak plasma concentration of paraceta- The administration of dopa decarboxylase inhi- mol and aspirin (Prescott, 1974; Volans, 1974). bitors, eg, carbidopa in Sinemet, or benserazide in Some drugs may interact chemically in the gut, Madopar, probably increases the quantity of leading to impaired absorption of both drugs. levodopa which reaches the systemic circulation Calcium, magnesium and aluminium ions contained unmetabolized, as well as reducing its peripheral in antacid preparations cause the formation of a metabolism in tissues which contain the enzyme, metal-tetracycline chelate which is poorly absorbed such as sympathetic nerve terminals. In this case the (Kunin and Finland, 1961). Iron salts given simul- interaction is used to advantage because the amount taneously for anaemia will have a similar effect of levodopa which has to be administered daily to (Neuvonen and Turakka, 1974). These interactions the Parkinsonian patient can be reduced to between may result in an inadequate serum concentration of a quarter and a fifth by the addition of the enzyme the antibiotic being reached, with consequent inhibitor. failure of therapy. Chemical interaction in the gut It is likely that long-term therapy with liver- appears to account also for the failure of griseoful- enzyme-inducing drugs such as barbiturates can vin's antifungal action in patients receiving a reduce the quantity of other drugs reaching the barbiturate drug (Riegelman, Rowland, and Epstein, systemic circulation by increasing their first-pass 1970), and the reduction in plasma rifampicin metabolism, although it is not easy to distinguish concentrations by concurrent administration of this type of interaction from one involving a reduc- (Boman, Hanngren, Malm- tion in absorption from the gut. para-aminosalicylate copyright. borg, Borga, and Sjoqvist, 1971). Although barbi- turates are known to induce the hepatic metabolism Plasma-protein-binding Interactions of oral anticoagulants, it has been suggested that an interaction in the gut lumen might also contribute Many drugs are loosely bound to plasma proteins, to the lowering of the serum level of the anti- usually albumin. Only the unbound drug molecules coagulant (O'Reilly and Aggeler, 1969). are available for pharmacological action, although A deficiency of vitamin K in a patient treated with as they are removed from the circulation by distri- oral anticoagulants will exaggerate the effect of bution or elimination they are replaced by other these latter compounds. Commensal organisms in molecules detaching themselves from protein- http://jcp.bmj.com/ the gut produce vitamin K and therefore gut binding sites. A number of drugs are extensively sterilization with broad-spectrum antibiotics may bound to plasma proteins and therefore competition disturb anticoagulant control. Ingestion of liquid for binding sites can be a cause of drug interaction, paraffin may have the same effect because of the fat- although it is not easy to predict which drugs are soluble nature of the vitamin. likely to interact in this way because different types of binding sites may be used by different drugs. On the

Interactions during Absorption whole, it is probable that competition for binding on September 29, 2021 by guest. Protected sites is not often a cause of a clinically important Drugs which are absorbed from the gut enter the interaction. portal venous system and are therefore exposed to Although displacement of a proportion of a drug drug-metabolizing enzymes in the hepatic cells bound to plasma proteins will increase the concen- before they enter the systemic circulation. This tration of free drug in the serum and therefore its results in a considerable loss of drugs that are pharmacological effect, the extent to which it is metabolized rapidly on first passage through the increased is dependent upon the drug's distribution liver-the so-called 'first pass effect'. Examples of volume. Centrally acting drugs, eg, phenytoin, are drugs for which this is important are isoprenaline, usually highly lipid soluble and have a large distri- ephedrine, levodopa, propranolol, aspirin and bution volume. A small quantity displaced from nortriptyline. Drug interactions may alter the extent binding sites will therefore rapidly redistribute to of first pass metabolism. This is likely to be most other tissues and will not cause an important important when a normally high rate of first pass increase in the effects of the drug. Warfarin, however, J Clin Pathol: first published as 10.1136/jcp.28.Suppl_9.94 on 1 January 1975. Downloaded from

96 Alan Richens is both highly bound to plasma proteins (98-99%) tion (by enzyme induction). This latter type of and has a relatively small distribution volume. interaction may account for the conflicting reports on Displacement from its binding sites by phenyl- the effect of phenobarbitone therapy on steady-state butazone (Aggeler, O'Reilly, Leong, and Kowitz, serum phenytoin levels in epileptic patients (Kutt, 1967) and chloral hydrate (Sellers and Koch-Weser, 1972). Inhibition of drug metabolism is of clinical 1971) may cause a sudden increase in the anti- importance when the pharmacological effect of the coagulant response with an attendant danger of drug is dangerous or toxic in excess. At serum levels bleeding. With phenylbutazone, however, inhibition above 20 or 25 ,ug/ml phenytoin frequently produces of the metabolism of warfarin may be more impor- symptoms of cerebellar dysfunction, namely, coarse tant than displacement from its binding sites (Lewis, nystagmus, ataxia and slurred speech. If sulthiame Trager, Chan, Breckenridge, Orme, Rowland, and is added as a supplementary antiepileptic drug, the Schary, 1974). serum concentration of phenytoin is likely to increase by an average of75 % (Houghton and Richens, 1974), Interactions at Site of Metabolism leading to phenytoin intoxication in a substantial proportion. It is possible that sulthiame has anti- Most drugs which are used therapeutically in man convulsant activity only by its ability to inhibit are metabolized by enzyme systems in the liver, the the metabolism of other drugs given in combination. object being to convert lipid-soluble drugs into more Because phenytoin metabolism is saturable it is polar water-soluble metabolites which can be easily inhibited. A number of other drugs can cause rapidly cleared by the kidneys. Many drugs use inhibition, such as isoniazid, pheneturide, disul- common microsomal enzyme systems, for example, firam, chloramphenicol, dicoumarol and phenyr- the cytochrome P450-dependent oxidase system by amidol (Kutt, 1972). which drugs are hydroxylated, dealkylated, deamin- Other examples of inhibition of drug metabolism ated or sulphoxidized. Substrates of the system, for are the increase in the anticoagulant action of example, phenobarbitone, phenytoin, primidone, warfarin by oxyphenbutazone (Fox, 1964), nor- glutethimide, rifampicin, can cause enzyme induction triptyline and various other drugs (Koch-Weser and ifthey are administered to repeatedly. Six eight weeks Sellers, 1971), and the hypoglycaemia which can copyright. of treatment is required for this effect to become result from adding dicoumarol (Kristensen and maximal. Not only is the metabolism of the inducing Hansen, 1967), chloramphenicol (Christensen and drug stimulated, but many other drugs and endo- Skovsted, 1969) or sulphaphenazole (Christensen, genous substances will be turned over much more Hansen, and Kristensen, 1963) to tolbutamide rapidly (Richens, 1974). This can lead to a reduction therapy in the diabetic patient. in the anticoagulant effect of dicoumarol (Hansen, Siersbaek-Nielsen, Kristensen, Skovsted, and Interaction at Site of Excretion Christensen, 1971), the anti-asthmatic effect of corticosteroids (Brooks, Werk, Ackerman, Sullivan, Many acidic drugs and their metabolites are secreted http://jcp.bmj.com/ and Thrasher, 1972), the contraceptive effect of the actively by the renal tubules, and competition for 'pill' (Laengner and Detering, 1974), the anti- secretion can occur. For example, probenecid depressant effect of nortriptyline (Alexanderson, competes with penicillin, greatly prolonging its Evans, and Sjoqvist, 1969; Braithwaite, Flanagan, plasma half-life. Similarly, phenylbutazone com- and Richens, 1975) and the antibiotic effect of petes with chlopropamide and acetohexamide, and doxycycline (Neuvonen and Penttila, 1974). may potentiate the hypoglycaemic effect of these

Perhaps even more important than these thera- drugs (Field, Ohta, Boyle, and Remer, 1967). on September 29, 2021 by guest. Protected peutic failures is the potential danger which is Weakly acidic drugs are cleared faster in an created when an enzyme-inducing drug is stopped in alkaline than in an acid urine, which is the reason for a patient receiving a second drug with a narrow promoting a forced alkaline diuresis in patients therapeutic ratio, eg, warfarin. A gradual reduction overdosed with aspirin or phenobarbitone. Adminis- in its rate of metabolism following withdrawal of a tration of carbonic-anhydrase-inhibited drugs, eg, barbiturate hypnotic can turn a therapeutic dose into acetazolamide, may have a similar action. a lethal one. In these circumstances the coagulability of the patient's blood will need to be carefully Pharmacodynamic Interactions monitored. Some drugs will inhibit, rather than induce the The interactions described above have been the metabolism of a second drug, and it appears that a result of a change in the kinetics of one drug by biphasic response is sometimes seen, first inhibition another. Some important interactions occur, how- (presumably by substrate competition) then stimula- ever, by a change in the sensitivity of a tissue to the J Clin Pathol: first published as 10.1136/jcp.28.Suppl_9.94 on 1 January 1975. Downloaded from

Drug interactions and lethal drug combinations 97 actions of a drug. A simple additive effect of central over the chemist's counter, and are also present in nervous system depressant drugs would be a good various foodstuffs (Sj6qvist, 1965). The following example. It has been said that nitrazepam will medicines should be strictly avoided in patients rarely, if ever, cause death when taken alone in receiving a monoamine oxidase inhibiter: overdosage, but when alcohol or a barbiturate is Cough mixtures, which often contain indirectly taken at the same time the suicidal attempt may be acting sympathomimetic amines as bronchodilators, successful. such as ephedrine or phenylpropanolamine; Drugs which modify the function of sympathetic Nasal decongestants, which usually contain nerve terminals can change the pharmacological ephedrine or phenylephrine. Proprietary 'cold cures' effects of other drugs acting on the system. For often contain similar substances. example, antihypertensive drugs which act by Anti-obesity preparations, which are usually blocking the release of noradrenaline from the indirectly acting sympathomimetic amines or related terminals, eg, guanethidine, bethanidine, debriso- drugs. quine, cause a 'denervation supersensitivity' of the Indirectly-acting sympathomimetic amines used receptor sites, leading to an exaggerated pressor for any other purposes, eg, amphetamine derivatives, response to directly acting sympathomimetic amines methylphenidate. such as adrenaline and noradrenaline (Dollery, Levodopa used for Parkinsonism. 1965). An enhanced pressor response is also seen In addition to the above drugs, tricyclic anti- with tricyclic antidepressant therapy, although in depressants and pethidine should be used with this case it is probably the result of an impaired great caution in patients receiving monoamine uptake of the amine into the sympathetic nerve oxidase inhibitors. The former are likely to produce terminal, leading to excessive concentrations at the excitation, hyperpyrexia, delirium and coma. Despite receptor sites. With both of these groups of drugs it is these alarming adverse effects, tricyclics are some- probably wise to avoid the use of local anaesthetic times cautiously used in combination with mono- agents containing adrenaline or noradrenaline as a amine oxidase inhibitors in patients with resistant vasoconstrictor during dentistry (Boakes, Laurence, depression. Pethidine can also produce alarming

Teoh, Barar, Benedikter, and Prichard, 1973). The effects resembling an overdosage of the drug, copyright. use of felypressin, a directly acting polypeptide probably because its metabolism is inhibited by vasoconstrictor, appears to be safe, however (Boakes monoamine oxidase inhibitors. Morphine and et al, 1973). pentazocine appear not to carry the same risks. Because tricyclic antidepressants block the uptake Certain foodstuffs contain indirectly acting mechanism into sympathetic neurones they reverse the sympathomimetic amines such as tyramine. The hypotensive effect of sympathetic neurone-blocking following have been reported to contain substantial drugs (Mitchell, Cavanaugh, Arias, and Oates, 1970). amounts of this amine and should therefore be This is probably a common cause of failure of anti- avoided: cheeses, yeast extracts, eg, Marmite, hypertensive therapy, particularly as these latter pickled herring, Chianti. Although there are many http://jcp.bmj.com/ drugs sometimes produce depression. Chlorproma- other foods which appear on warning lists, there is zine can also block the effects of these drugs (Fann, no clear evidence with most that a hazardous inter- Janowsky, Davis, and Oates, 1971), as can indirectly action can occur. acting sympathomimetics contained in cough mix- It should be noted that directly acting sympatho- tures (Misage and McDonald, 1970). mimetic amines do not interact with monoamine The dangerous interactions between monoamine oxidase inhibitors, in contrast to the tricyclic

oxidase inhibitors and various drugs and food- antidepressants (Boakes et al, 1973). on September 29, 2021 by guest. Protected stuffs are well known. The antidepressant effect of Various other interactions can occur at a tissue these compounds is thought to result from an level which in broad terms can be described as accumulation of noradrenaline and 5-HT in mono- pharmacodynamic interactions. The most important amine-producing nerve terminals in the brain. A of these are the potentiation of cardiac glycosides by similar effect occurs in peripheral sympathetic thiazide-induced hypokalaemia, the exaggerated nerve terminals resulting in a greater store of hypoglycaemia produced by propranolol in diabetic transmitter available for release by nerve impulses patients treated with oral agents, and the antagon- or by indirectly acting sympathomimetic amines. ism of these latter agents by thiazide diuretics, Interaction with these latter compounds will cause a corticosteroids, and the contraceptive pill. hypertensive crisis, which may end fatally. This constitutes the chief danger of monoamine oxidase- Conclusion inhibiting therapy because these amines are con- tained in proprietary medicines which can be bought The interactions which have been mentioned in this J Clin Pathol: first published as 10.1136/jcp.28.Suppl_9.94 on 1 January 1975. Downloaded from

98 Alan Richens paper are only a selected few of those that have been Field, J. B., Ohta, M., Boyle, C., and Remer, A. (1967). Potentiation of acetohexamide hypoglycemia by phenylbutazone. New reported. They have been chosen for special mention Engl. J. Med., 277, 889-894. because they are among the more serious that can Fox, S. L. (1964). Potentiation of anticoagulants caused by pyrazole occur Unfortunately, the lists of potential compounds. J. Amer. med. Ass., 188, 320-321. clinically. Hansen, J. M., SiersbEek-Nielsen, K., Kristensen, M., Skovsted, L., interactions are growing longer each day, partly and Christensen, L. K. (1971). Effect of diphenylhydantoin on because we are becoming more aware of this the metabolism of dicoumarol in man. Acta med. scand., 189, 15-19. problem as a real risk in clinical practice, but partly Houghton, G. W., and Richens, A. (1974). Phenytoin intoxication because lists are compiled by one author, added to induced by sulthiame in epileptic patients. J. Neurol. Neurosurg. Psychiat., 37, 275-281. by another, recompiled by a third and so on, with Hurwitz, N., and Wade, 0. L. (1969). Intensive hospital monitoring the result that the inference from a single case report of adverse reactions to drugs. Brit. med. J., 1, 531-536. may become perpetuated as if it were a proven Koch-Weser, J., and Sellers, E. M. (1971). Drug interactions with coumarin anticoagulants. New Engi. J. Med., 285, 487-498; interaction. Reference back to the original source can 547-558. often be quite revealing. Kristensen, M., and Hansen, J. M. (1967). Potentiation of the tolbutamide effect by dicoumarol. Diabetes, 16, 211-214. No physician can remember long lists. Fortunately, Kunin, C. M., and Finland, M. (1961). Clinical pharmacology of the this is unnecessary if the principles underlying drug tetracycline antibiotics. Clin. Pharmacol. Ther., 2, 51-69. Kutt, H. (1972). Diphenylhydantoin interactions with other drugs in interactions are understood, and the broad groups man. In Antiepileptic Drugs, edited by D. M. Woodbury, of drugs likely to interact in one of the ways descri- J. K. Penry, and R. P. Schmidt, pp. 169-180. Raven Press, bed in this paper are borne in mind. Nevertheless, New York. Laengner, H., and Detering, K. (1974). Antiepileptic drugs and failure there is a clear need for improving the effectiveness of oral contraceptives. (Letter) Lancet, 2, 600. of communication of information about drug inter- Lewis, R. J., Trager, W. F., Chan, K. K., Breckenridge, A., Orme, M., Rowland, M., and Schary, W. (1974). Warfarin-stereo- actions if unnecessary iatrogenic disease is to be chemical aspects of its metabolism and the interaction with avoided (Petrie, Howie, and Durno, 1974) and this phenylobutazone. J. clin. Invest., 53, 1607-1617. one the future roles Misage, J. R., and McDonald, R. H. (1970). Antagonism of hypo- is, perhaps, of of the clinical tensive action of bethanidine by 'common cold' remedy. Brit. pharmacologist. med. J., 4, 347. Mitchell, J. R., Cavanaugh, J. H., Arias, L., and Oates, J. A. (1970). References Guanethidine and related agents. III. Antagonism by drugs which inhibit the norepinephrine pump in man. J. clin. Invest., 49, 1596-1604. Aggeler, P. M., O'Reilly, R. A., Leong, L., and Kowitz, P. E. (1967). copyright. Potentiation of anticoagulant effect of warfarin by phenyl- Neuvonen, P. J., and Penttila, 0. (1974). Interaction between doxy- butazone. New Engl. J. Med., 276, 496-501. cycline and barbiturates. Brit. med. J., 1, 535-536. Alexanderson, B., Evans, D. A. P., and Sjoqvist, F. (1969). Steady- Neuvonen, P. J., and Turakka, H. (1974). Inhibitory effect of various state plasma levels of nortriptyline in twins: influence ofgenetic iron salts on the absorption of tetracycline in man. Europ. factors and drug therapy. Brit. med. J., 4, 764-768. J. clin. Pharmacol., 7, 357-360. Boakes, A. J., Laurence, D. R., Teoh, P. C., Barar, F. S. K., Benedik- O'Reilly, R. A., and Aggeler, P. M. (1969). Effect of barbiturates on ter, L. T., and Prichard, B. N. C. (1973). Interactions between oral anticoagulants in man. Clin. Res., 17, 153. sympathomimetic amines and antidepressant agents in man. Petrie, J. C., Howie, J. G. R., and Durno, D. (1974). Awareness and Brit. med. J., 1, 311-315. experience of general practitioners of selected drug interaction Boston Collaborative Drug Surveillance Program (1972). Adverse Brit. med. J., 2, 262-264. drug interactions. J. Amer. med. Ass., 220, 1238-1239. Prescott, L. F. (1974). Gastric emptying and drug absorption. Brit.

Boman, G., Hanngren, A., Malmborg, A. S., Borga, O., and J. clin. Pharmacol., 1, 189-190. http://jcp.bmj.com/ Sjoqvist, F. (1971). Drug interaction: decreased serum con- Richens, A. (1974). The clinical consequences of chronic hepatic centrations of rifampicin when given with P.A.S. Lancet, 1, enzyme induction by anticonvulsant drugs. Brit. J. clin. 800. Pharmacol., 1, 185-187. Braithwaite, R. A., Flanagan, R. J., and Richens, A. (1975). Steady- Riegelman, S., Rowland, M., and Epstein, W. L. (1970). Griseofulvin- state plasma nortriptyline concentrations in epileptic patients. phenobarbital interaction in man. J. Amer. med. Ass., 213, 426- Brit. J. clin. Pharmacol., in press. 431. Brooks, S. M., Werk, E. E., Ackerman, S. J., Sullivan, I., and Thrasher, Shapiro, S., Slone, D., Lewis, G. P., and Jick, H. (1971) Fatal drug K. (1972). Adverse effects of phenobarbital on corticosteroid reactions among medical in-patients. J. Amer. med. Ass., 216, metabolism in patients with bronchial asthma. New Engl. J. 467-472. Med., 286, 1125-1128. Sellers, E. M., and Koch-Weser, J. (1971). Kinetics and clinical Christensen, L. M., Hansen, J. M., and Kristensen, M. (1963). importance of displacement of warfarin from albumin by on September 29, 2021 by guest. Protected Sulphaphenazole-induced hypoglycaemic attacks in tolbuta- acidic drugs. Ann. Amer. N.Y. Acad. Sci., 179, 213-225. mide-treated diabetics. Lancet, 2, 1298-1301. Sjoqvist, F. (1965). Psychotropic drugs. 2. Interaction between Christensen, L. K., and Skovsted, L. (1969). Inhibition of drug monoamine oxidase (MAO) inhibitors and other substances. metabolism by chloramphenicol. Lancet, 2, 1397-1399. Proc. roy. Soc. Med., 58, 967-978. Dollery, C. T. (1965). Physiological and pharmacological interactions Smith, J. W., Seidl, L. G., and Cluff, L. E. (1966). Studies on the of antihypertensive drugs. Proc. roy. Soc. Med., 58, 983-987. epidemiology of adverse drug reactions. V. Clinical factors Fann, W. E., Janowsky, D. S., Davis, J. M., and Oates, J. A. (1971). influencing susceptibility. Ann. intern. Med., 65, 629-640. Chlorpromazine reversal of the antihypertensive action of Volans, G. N. (1974). Absorption of effervescent aspirin during guanethidine. (Letter) Lancet, 2, 436-437. migraine. Brit. med. J., 4, 265-268.