Drug interactions are defined as the alteration of the effects of one drug by the prior or concurrent administration of another and the usual result is an increase or decrease of the effects of one of the drugs.

Desired interactions are usually considered in the context of “combination therapy” (for example, in the treatment of raised blood pressure, asthma, certain infections and malignant tumors), in which two or more drugs are used to increase the therapeutic effects and/or reduce the toxicity of drugs. Unwanted interactions can cause side effects or the ineffectiveness of the drugs.

Relatively few of the known or suggested drug interactions have been sufficiently analyzed to determine their clinical significance. If an interaction appears likely, a doctor will consider prescribing alternatives.

Drug interactions include the concurrent administration of drugs having the same (or opposing) pharmacological actions as well as alteration of the sensitivity or the responsiveness of the tissues to one drug or another. Many of these interactions can be predicted from a knowledge of the effects of each drug, and by monitoring patients, doctors will be able to detect deviations from expected effects and dosages can be adjusted accordingly.

Interactions that occur because of the way a drug passes through the body are more complicated and difficult to predict because the interacting drugs have unrelated actions. The interactions are mainly due to the processes of absorption, distribution, metabolism and excretion; the type of response expected from the drug is not changed, only the magnitude and duration.

The following general points concerning drug interactions warrant emphasis. (1) The drugs for which interactions are most significant are those with potent effect and low safety margins, such as
- anticoagulant drugs (used to reduce clotting of the blood);
- digitalis preparations (used in the treatment of heart failure);
- cytotoxic medicines (used to treat cancer);
- drugs for the treatment of hypertension (high blood pressure);
- drugs for the treatment of low blood
sugar.

(2) It may be difficult to distinguish a drug interaction from symptoms of the illness or disease that can affect the body’s response to a drug.

(3) Not all patients develop reactions, even when it is known that interactions may occur. Individual factors, such as dose and metabolism, determine whether the phenomenon occurs.

(4) When the effects of drugs are being closely monitored, an interaction usually requires a change of dosage or drug and does not result in significant problems for the patient.
Avoidance ofdrur; interaction

To minimize the incidence and consequences of drug interactions, doctors should adhere to a number of general principles, and patients themselves should help doctors in detecting the symptoms of drug interaction.

(1) Doctors should know their patients’ total drug intakes, including all agents prescribed by others and those that are purchased without a prescription. Patients should honestly tell their doctors about all chemicals and other agents they are using.

(2) Doctors should prescribe as few drugs in as low doses as possible for as short a time as needed to achieve a desired effect, and should avoid unnecessary combinations.

(3) Doctors should know the effects, both wanted and unwanted, of all the drugs used (since the spectrum of drug interactions is usually contained within these effects) and know which doses produce which responses.

(4) Doctors should observe and monitor their patients for the drugs’ effects, particularly after any alteration in therapy.

(5) Doctors should, with the aid of accurate information from their patients, consider drug interactions as possible causes of any unanticipated trouble. If unexpected responses do occur, blood levels of drugs being taken should be measured, if possible. Most importantly, the doses of drugs should be altered until the desired effect is obtained. If this fails, the drugs should be changed to alternatives that will not interact with others being taken.