Where and How Drug Interaction Occurs?

Where and How Drug Interaction Occurs?

It is usual for patients to receive a number of drugs at the same time. There are reports of getting 36 different drugs in one admission at John Hopkins Hospital in the United States. A researcher described this as a carrier bag syndrome. When a drug is administered, a response is obtained; if a second drug is given and the response to the first drug is altered, a drug interaction is said to have occurred. Thus, drug interaction is a phenomenon which occurs when the effects of one drug are modified by the prior or concurrent administration of another drug(s). A drug interaction may result in beneficial or harmful effects. However, harmful effects are usually predominated.


Drugs can interact with one another at any point from their being mixed in a pharmaceutical formulation up to their final elimination from the body. Thus, interactions can occur a) outside the body, b) during absorption, c) during distribution, d) at the receptor level (pharmacodynamics) e) during biotransformation and f) during excretion

Interactions outside the body:

Serious loss of potency can occur from incompatibility between an infusion fluid and a drug that is added to it. Similarly, addition of more than one drug to the same infusion fluid may result in interactions causing loss of activity. The immediate effect of soluble insulin is reduced if it is drawn up with potassium zinc insulin in the same syringe or drip.

Mixture----------------------------------: Results

Thiopentone + Suxamethonium--------- Precipitation
Diazepam + Infusion fluid----------- Precipitation
Phenytoin + Infusion fluid-----------Precipitation
Heparin + Hydrocortisone-------------Inactivation of heparin
Kanamycin + Hydrocortisone-----------Inactivation of kanamycin
Carbenicillin + Gentamicin-----------Inactivation of Gentamicin

Interactions during absorption:

Drugs may interact in the gastrointestinal tract resulting in either decreased or increased absorption. Altered absorption may be due to any of the following mechanisms:

Direct chemical interactions between drugs:

Antacids containing aluminum, magnesium or calcium form chelation with tetracycline’s and reduced bioavailability of tetracycline. Similar interactions occur between iron and tetracycline. Furthermore, cholestyramine interferes with absorption of thyroxine, digoxin, and warfarin by the process of adsorption.

Altering gastrointestinal motility:

Alteration of gastrointestinal motility influences the rate and extent of drug absorption. Antichoiinergic drugs and metoclopramide alter the bioavailability of many drugs by decreasing and increasing gastrointestinal motility respectively.

Mixture--------------------------------Mechanism----------------Results

Tetracycline + Sodium bicarbonate---- Altered pH---Decreased absorption of tetracycline
Tetracycline + Calcium ---------------Chelation ---Decreased absorption of tetracycline
Tetracycline + Magnesium -------------Chelation----Decreased absorption of tetracycline
Tetracycline + Aluminum --------------Chelation ---Decreased absorption of tetracycline
Tetracycline + Iron ------------------Chelation ---Decreased absorption of tetracycline
Digitalis + Cholestyramine-------Complex formation----Decreased absorption of Digitalis
Warfarin + Cholestyramine--------Complex formation --Decreased absorption of Warfarin
Thiazide + Cholestyramine--------Complex formation ---Decreased absorption of Thiazide

Interactions during distribution:

Displacement from plasma protein binding sites:

A drug which is extensively bound to protein can be displaced from its binding site by another drug which has greater affinity for the same binding site, sp raising free concentration of displaced drug. Drug like aspirin competes with warfarin for the same protein binding site. Aspirin displaces warfarin from binding site resulting in increased adverse effects of warfarin. Other examples are given below:

Bound drug---------- Displacing drug------------------Result

Bilirubin--------------Sulfonamide----------------- Kernicterus
Bilirubin--------------Vitamin K-------------------Kernicterus
Tolbutamide------------Salicylate------------------Hypoglycemia
Methotrexate------------Salicylate----------------- Agranulocytosis
Methotrexate------------Sulfonamide----------------- Agranulocytosis
Warfarin----------------Clofibrate--------------------Kernicterus
Sulfonamide-------------Salicylate----------------- Sulfonamide toxicity

Displacement from other tissue binding sites:

When quinidine is given to patients who are receiving digoxin, the plasma concentration of free digoxin is greatly increased because quinidine displaces digoxin from binding sites in the tissues as well as from plasma proteins.

Interactions during biotransformation:

Enzyme induction:

Enzyme induction by drugs accelerates biotransformation of drugs and is a cause of therapeutic failure. If the drug A is metabolized by the microsomal enzymes, then concurrent administration with a microsomal inducer (drug B) will result in enhanced metabolism of drug A. Anticoagulant control with warfarin is dependent on a steady state of elimination by metabolism.

Enzyme induction leads to accelerated metabolism of warfarin, loss of anticoagulant control and danger of therapeutic failure. In contrast, if a patient's anticoagulant control is stable on warfarin plus a inducing agent by adjustment of danger of hemorrhage then the inducing agent is discontinued because warfarin will be eliminated at a slower rate. Other examples are given below:

Primary drug-------Inducing drug--------------Result

Warfarin------------barbiturate---------- Decreased anticoagulation
Contraceptives------ Rifampicin----------- Failure of contraception
Quinidine------------Phenytoin------------ Reduced quinidine effect

Enzyme inhibition:

Enzyme inhibition potentiates other drugs whose intensity and duration of action are limited by being metabolized. For example, warfarin metabolism is inhibited by an enzyme inhibitor cimetidine. Thus, cimetidine alters the optimal therapeutic concentrations that were stabilized by a dose of warfarin, resulting in hemorrhage. Other examples are given below:

Primary drug--------Inhibiting agent--------------Result

Warfarin------------ Metronidazole--------------Hemorrhage
Phenytoin----------- Isoniazid----------- Phenytoin toxicity
Azothioprine--------- Allopurinol---------Bone marrow suppression

Interactions during excretion:

Interference with active transport:

Probenecid competes with penicillins for active transport process at the kidney and prolongs the action of penicillins, which is beneficial. Other examples are given by chart:

Primary drug------------Competing drug--------------Result

Salicylate------------- Probenecid------------ Salicylate toxicity
Methotrexate------------Sulfonamide-------- Increased methotrexate level
Digoxin---------------- Spironolactone--------- Digoxin toxicity

Interference with passive diffusion:

Alteration of urinary pH influences ionization of drugs and their excretion. Thus, basic drugs are better excreted in acidic urine and acidic drugs are better excreted in alkaline urine. Enhanced excretion .of aspirin occurs if sodium bicarbonate is given. Similarly rapid excretion of amphetamine is achieved by giving ammonium chloride.

Pharmacodynamic drug interactions:

It’s quite natural to use two or more drugs at the same time. The use of two or more may produce either no effect, synergism or antagonism.

Drug synergism: When the therapeutic or toxic effects of two drugs are greater than the effect of individual drug, it is synergism. Drug synergism is of two types- additive effect or potentiation.

Additive effect: When the net effect of two drugs used together is equal to the sum of the individual drug effects, the drugs are said to have an additive effect. For example, the combination of a thiazide diuretic and a beta adrenergic blocking drug is used for the treatment of hypertension.

Potentiation: When the net effect of two drugs used together is greater than the sum of the individual drug effects, the drugs are said to have potentiation effect. For example, the combination of sulfamethoxazole and trimethoprim is used as antimicrobial agents.

Drug antagonism: The effects of one drug can be reduced or abolished by the presence of another drug and this effect is termed drug antagonism. Drug antagonism is of three types- chemical, physiological and pharmacological. Physiological and pharmacological antagonisms involve an interaction of an agonist and an antagonist.

Chemical antagonism: When a drug antagonizes the effect of another drug by simple chemical reaction without action on the receptor. For example, antacid neutralizes the gastric acid.

Physiological antagonism: When the physiological effect of a drug is antagonized by another drug by acting on two different types of receptors. For example, acetylcholine causes contraction of intestinal smooth muscle by acting on muscarinic cholinoceptors. Whereas this action of acetylcholine is antagonized (that is relaxation of the intestinal smooth muscle) by adrenaline.

Adrenaline acts by interacting on adrenoceptors. Noradrenaline contracts vascular smooth muscle to increase blood pressure, whereas histamine relaxes vascular smooth muscle to decrease blood pressure. Acetylcholine causes constriction whereas adrenaline causes dilatation of the pupil.

Pharmacological antagonism: When a drug antagonizes the effect of another drug by acting on the same receptor it is called pharmacological antagonism. Pharmacological antagonism is of two types- competitive and noncompetitive.

Competitive antagonism: Competitive antagonism is reversible. The inhibitory effect of an antagonist is overcome by-using large amount of agonist. Here, both, the agonist and antagonist compete for the same receptor and are able to displace each other at the receptor site. For example, acetylcholine causes contraction of intestinal smooth muscle. Atropine blocks this effect of acetylcholine. When a low concentration of an agonist is plotted against, the responses, a sigmoid shaped curve will be obtained. The maximum response of the agonist is obtained in the presence of competitive antagonist. The dose-response curve remains parallel but is shifted to the right.

Noncompetitive antagonism: The maximum response of an agonist in the presence of antagonist is reduced. The inhibitory effect of a drug is not overcome by using large amount of agonist In this case the antagonist acts at some rate-limiting step of the response, distal to the drug-receptor complex.

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