Drug Interactions

Some knowledge of how one drug may change the action of another is useful.

Interactions may result in antagonism as well as synergism e.g. a betablocker and a thiazide have additive antihypertensive effect-wanted interactions and in recognizing and preventing that are undesirable.

Antagonism occurs when two drugs have opposite pharmacological effects e.g. histamine and adrenaline or they compete for the same drug receptor (e.g. isopropyl noradrenaline is bronchodiolator and propranolol - a betablocker).

Combination of drugs may produce effect greater then the combined effect of the two agents e.g. 2 + 2 = 5 e.g. trimethoprim and sulphonamide or even 0 + 2 = 5 e.g. benserazide plus levodopa. Drugs can interact even in vitro - stage when these are mixed with other drugs in IV infusion or a syringe or during absorption from the gut or during distribution.

Many patients, specially the elderly, are treated with combination of one or more drugs as this population may suffer from more than one disease.

In treatement of acute events such as severe infections, myocardial infarction also requires multiple drugs.

Therefore potential for drug interactions is substantial. Drugs can also interact with dietary constituents (e.g. graperfruit juice that down regulates isoforms of P 450 CYP 3 A4 in the gut) and herbal remedies (St.

John's wort). Drugs with steep concentration response curve and narrow therapeutic margin can cause major problems e.g. with antithrombotic, antiarrhythmic and antiepileptic drugs, lithium and several antineoplastic immunsuppressant drugs.

This book presents the reader with information on drug interactions and also provide suggestions to help improve prescription.