The effect a drug has on a person may be different than expected because that drug interacts with
Another drug the person is taking (drug-drug interaction)
Food, beverages, or supplements the person is consuming (drug-nutrient interaction)
Another disease the person has (drug-disease interaction)
The effects of drug interactions are usually unwanted and sometimes harmful. Interactions may
Increase the actions of one or more drugs, resulting in side effects or toxicity
Decrease the actions of one or more drugs, resulting in failed treatment
Drug-Drug Interactions
Drug-drug interactions can involve prescription or nonprescription (over-the-counter) drugs. Types of drug-drug interactions include duplication, opposition (antagonism), and alteration of what the body does to one or both drugs.
Duplication
When two drugs with the same effect are taken, their side effects may be intensified. Duplication may occur when people inadvertently take two drugs (often at least one is an over-the-counter drug
Similar problems with duplication can arise when two different drugs with the same effect are taken. This is most likely to occur when people see several doctors, obtain prescriptions at more than one pharmacy, or both. Doctors who are not aware of what others have prescribed may inadvertently prescribe similar drugs. For example, excessive sedation and dizziness can occur when two doctors both prescribe a sleep aid or when one prescribes a sleep aid and the other prescribes another drug (such as an antianxiety drug) that has similar sedative effects.
People can reduce the risk of this kind of duplication by keeping each doctor informed about all drugs being taken and by using one pharmacy to obtain all prescriptions. It is helpful to keep an up-to-date written list of all drugs being taken and to bring the list along on each doctor visit. Also, people should not take previously prescribed drugs (such as a sleeping pill or pain reliever) without checking with the doctor or pharmacist because that drug may duplicate or otherwise interact with one of their current drugs.
Opposition (antagonism)
Two drugs with opposing actions can interact, thereby reducing the effectiveness of one or both. For example, nonsteroidal anti-inflammatory drugsTargets in The Body: Cell Receptors)—but one type blocks them, and the other stimulates them.
Alteration
One drug may alter how the body absorbs, distributes, metabolizes, or excretes another drug (see Administration and Kinetics of Drugs).
Because there are so many drug interactions, many doctors and pharmacists reduce the risk of problems by checking reference books and computer software programs when prescribing or dispensing prescriptions for additional drugs. In most pharmacies, drug orders and prescriptions are reviewed using a computer system that automatically checks for drug interactions.
Drug-Nutrient Interactions
Nutrients include food, beverages (including alcohol), and dietary supplements. Consumption of these substances may alter the effects of drugs the person takes.
Food
Like food, drugs taken by mouth must be absorbed through the lining of the stomach or the small intestine. Consequently, the presence of food in the digestive tract may reduce absorption of a drug. Often, such interactions can be avoided by taking the drug 1 hour before or 2 hours after eating.
Dietary supplements
Dietary supplements, including medicinal herbs, are products (besides tobacco) that contain, for example, a vitamin, mineral, herb, or amino acid and that are intended as a supplement to the normal diet. Supplements are regulated as foods, not as drugs, so they are not tested as comprehensively. However, they may interact with prescription or over-the-counter drugs. People who take dietary supplements should tell their doctors and pharmacists, so that interactions can be avoided.
Alcohol
Drug-Disease Interactions
Sometimes, drugs that are helpful in one disease are harmful in another disorder. For example, some beta-blockers taken for heart disease or high blood pressure can worsen asthma or make it hard for people with diabetes to tell when their blood sugar is too low. Some drugs taken to treat a cold may worsen glaucoma. People should tell their doctor all of the diseases they have before the doctor prescribes a new drug. Diabetes, high or low blood pressure, an ulcer, glaucoma, an enlarged prostate, poor bladder control, and insomnia are particularly important, because people with such diseases are more likely to have a drug-disease interaction.
Drug-disease interactions can occur in any age group but are common among older people, who tend to have more diseases (see Aging and Drugs).
