Drug Interactions

When you take two or more drugs, there is a chance that it would be the interaction between them. Interaction may increase or decrease the effectiveness of drugs or drug side-effects. The probability of drug interactions with increasing number of drugs taken increases. Therefore, people who take several drugs are in most danger. The survey focuses on the interaction of drugs and ways to avoid them.

What are drug interactions?

A drug interaction can be defined as an interaction between a drug and another substance that prevents the drug from performing as expected. This definition applies to interactions of drugs with other drugs , as well as drugs with food  and any other substances.

How do drug interactions occur?

There are several mechanisms by which drugs interact with other drugs, food, or any other substances. An interaction can result when :

  1. the absorption of a drug into the body;
  2. distribution of the drug within the body;
  3. alterations made to the drug by the body (metabolism); and
  4. elimination of the drug from the body.

Drug interactions also may occur when two drugs that have similar  effects or opposite effects on the body are taken together. For example, there may be major sedation when two drugs that have sedation as side effects are given, for example, narcotics and antihistamines.

Another source of drug interactions occurs when one drug alters the concentration of a substance that is normally present in the body. The alteration of this substance reduces or enhances the effect of another drug that is being taken.

Most drugs are absorbed into the blood and then travel to their site of action. Most drug interactions that are due to altered absorption occur in the intestine. There are various potential mechanisms through which the absorption of drugs can be reduced:

  1. an alteration in blood flow to the intestine;
  2. change in drug metabolism (breakdown) by the intestine;
  3. increased or decreased intestinal movement;
  4. alterations in stomach acidity, and
  5. a change in the bacteria that reside in the intestine.

Drug absorption also can be affected if the drug’s ability to dissolve  is changed by another drug or if a substance, for example food,  binds to the drug and prevents its absorption.

Most drugs are eliminated through the kidney in either an unchanged form or as a by-product that results from the metabolism of the drug by the liver. Therefore, the kidney and the liver are very important sites of potential drug interactions. Some drugs are able to reduce or increase the metabolism of other drugs by the liver or their elimination by the kidney.

Drug interactions may lead to an increase or decrease in the beneficial or the adverse effects of the given drugs. When a drug interaction increases the benefit of the administered drugs without increasing side effects, both drugs may be combined to increase the control of the condition that is being treated.

For example, drugs that reduce blood pressure by different mechanisms may be combined because the blood pressure lowering effect achieved by both drugs may be better than with either drug alone.

The absorption of some drugs is increased by food. Therefore, these drugs are taken with food in order to increase their concentration in the body and, ultimately, their effect. Conversely, when a drug’s absorption is reduced by food, the drug is taken on an empty stomach.

Drug interactions that are of greatest concern are those that reduce the desired effects or increase the adverse effects of the drugs. Drugs that reduce the absorption or increase the metabolism or elimination of other drugs tend to reduce the effects of the other drugs. This may lead to failure of therapy or warrant an increase in the dose of the affected drug. Conversely, drugs that increase absorption or reduce the elimination or metabolism of other drugs – increase the concentration of the other drugs in the body – and lead to increased amounts of drug in the body and more side effects. Sometimes, drugs interact because they produce similar side effects. When two drugs that produce similar side effects are combined, the frequency and severity of the side effect are increased.

The prescribing information for most drugs contains a list of potential drug interactions. Many of the listed interactions may be rare, minor, or only occur under specific conditions and may not be important. Drug interactions that cause important changes in the action of a drug are of greatest concern.