by Sudesh Samuel

Created on: January 01, 2009

A drug-drug interaction involves one drug affecting the activity of another usually when both are concurrently administered. Such interactions can result in the reduced or enhanced activity of one or both drugs. While many forms of drug-drug interactions exist, most can be broadly classified into interactions that involve the travel of the drug once administered (pharmacokinetic) or the action of the drug on the body (pharmacodynamic).

Drug-drug interactions that affect the travel of drugs in the body can be further understood as those that affect absorption into the bloodstream, distribution within the body, breakdown into different products or removal from the body.

Absorption interactions can occur when one drug's particles have a large enough surface area to cause another drug's particles to stick to them, both drugs may also bind to each other, or one drug alters the acidity of the stomach contents or the rate at which the stomach moves it's contents. These interactions can alter the ability of one or both drugs to get into the bloodstream. When a drug only reduces the rate of absorption of another, a patient on regular use of both drugs is usually unaffected. However, if one drug reduces the extent of absorption of the other, the patient can be exposed to lower levels of the second drug than required and the second drug may hence be ineffective in treatment.

Distribution interactions may occur when drugs reach the bloodstream and the tissues. Competition between two drugs can arise for binding to the same proteins in the blood or one drug may dislodge another from it's connection with tissues. It is more common in the second instance when one drug displaces another from the tissues, to find that the displaced drug accumulates in the blood leading to a greater risk of an affected patient experiencing toxicity. An example is when the heart medications quinidine and Lanoxin (digoxin) are taken concurrently, digoxin blood levels can rise and adversely affect the patient if not monitored appropriately.

While drugs can be broken down at many different sites in the body, the most common site is the liver. Here, a system of enzymes can be up-regulated or down-regulated by one drug to result in the quicker or slower breakdown of the other respectively. Examples of drugs that up-regulate specific enzymes in the liver, include the anti-epileptics Dilantin (phenytoin) and Tegretol (carbamazepine). The enzyme up-regulation effect usually takes place gradually with maximal

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