Drug Interactions

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 Overview of Over-the-Counter Drugs Over-the-counter (OTC) drugs are those available without a prescription. OTC drugs enable people to relieve many annoying symptoms and to cure some diseases simply and without the cost of seeing... read more ) that have the same active ingredient. For example, people may take a cold remedy and a sleep aid, both of which contain diphenhydramine, or a cold remedy and a pain reliever, both of which contain acetaminophen. This type of duplication is particularly likely with the use of drugs that contain multiple ingredients or that are sold under brand names (thus appearing to be different but actually containing the same ingredients).

Awareness of drug ingredients is important, as is checking each new drug to avoid duplication. For example, many prescription-strength pain relievers contain an opioid plus acetaminophen. People taking such a product who do not know its ingredients might take over-the-counter acetaminophen for extra relief, risking toxicity.

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.

Two drugs with opposing actions can interact, thereby reducing the effectiveness of one or both. For example, nonsteroidal anti-inflammatory drugs Nonsteroidal Anti-Inflammatory Drugs Pain relievers (analgesics) are the main drugs used to treat pain. Doctors choose a pain reliever based on the type and duration of pain and on the drug's likely benefits and risks. Most pain... read more (NSAIDs), such as ibuprofen, which are taken to relieve pain, may cause the body to retain salt and fluid. Diuretics, such as hydrochlorothiazide and furosemide, help rid the body of excess salt and fluid. If a person takes both types of drug, the NSAID may reduce the diuretic's effectiveness. Certain beta-blockers (such as propranolol), taken to control high blood pressure and heart disease, counteract beta-adrenergic stimulants, such as albuterol, taken to manage asthma. Both types of drugs target the same cell receptors—beta-2 receptors (see table Targets in The Body: Cell Receptors Targets in The Body: Cell Receptors )—but one type blocks them, and the other stimulates them.

One drug may alter how the body absorbs, distributes, metabolizes, or excretes another drug (see Administration and Kinetics of Drugs Introduction to Administration and Kinetics of Drugs See Nanomedicine for an overview of all the ways in which nanotechnology has improved drug delivery. Drug administration is the giving of a drug by one of several means (routes). Drug kinetics... read more ).

Acid-blocking drugs, such as histamine-2 (H2) blockers and proton pump inhibitors, raise the pH of the stomach and decrease absorption of some drugs, such as ketoconazole, a drug for fungal infections.

Many drugs are broken down and inactivated (metabolized) by certain enzymes in the liver. Some drugs affect these liver enzymes, either increasing or decreasing their activity, and may cause another drug to be inactivated more quickly or more slowly than usual. For example, by increasing the activity of liver enzymes, barbiturates such as phenobarbital cause the anticoagulant warfarin to be inactivated more quickly and thus to be less effective when taken during the same time period. Conversely, by decreasing the activity of the enzyme system, drugs such as erythromycin and ciprofloxacin can increase the activity of warfarin, risking bleeding. When drugs that affect liver enzymes are used in people taking warfarin, doctors monitor the people more closely and adjust the dose of warfarin to compensate for this effect. The warfarin dose is adjusted again when other drugs are stopped. Many other drugs affect liver enzymes.

Chemicals in cigarette smoke can increase the activity of some liver enzymes. As a result, smoking decreases the effectiveness of some drugs, including theophylline (a drug that widens the airways called a bronchodilator).

Some drugs affect the rate at which the kidneys excrete another drug. For example, large doses of vitamin C increase the urine's acidity and thus may change the rate of excretion and activity of certain drugs. For example, the rate of excretion may be decreased for acidic drugs such as aspirin but may be increased for basic drugs such as pseudoephedrine.

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.

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 Overview of Dietary Supplements Dietary supplements are used by about 75% of Americans. They are the most common therapies included among integrative medicine and health (IMH) and complementary and alternative medicine (CAM)... read more , 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.

Although many people do not consider alcohol a nutrient, it affects body processes and interacts with many drugs. For example, taking alcohol with the antibiotic metronidazole can cause flushing, headache, palpitations, and nausea and vomiting. Doctors or pharmacists can answer questions about possible alcohol and drug interactions.