Drug Side Effects: When Your Medicine Is to Blame

Medically Reviewed

Bad reactions to medications result in more than 700,000 visits to hospital emergency rooms each year, according to the Centers for Disease Control and Prevention. Used inappropriately or incorrectly, prescription drugs may not help a patient very much and can cause harm.

And even though drugs are tested to earn approval from the Food and Drug Administration, they can cause unanticipated problems. For example, postmenopausal estrogen, after decades of immense popularity, tumbled from favor when a large, long-term study showed prolonged hormone therapy raised women’s risk of breast cancer, heart disease and stroke.

Some unexpected consequences of medications may be more bizarre than dangerous. For instance, a relatively small number of people taking the insomnia drug zolpidem (Ambien) experience strange nocturnal behavior. Some users arise during the night and, while unaware of doing so, walk or eat uncontrollably. Incredibly, some even drive their cars.

Drug side effects can range from unpleasant to potentially harmful. Whenever you start a new medication, you should be aware that side effects might occur, and you should know what to do if you think you’re having one or more of them. This is especially true for older adults, who are at greater risk of experiencing drug side effects.

What exactly are drug side effects?

A drug side effect is simply an unwanted effect of a medication. Side effects are often predictable, based on how the drug functions in the body. An example is low blood sugar (hypoglycemia) due to insulin and certain other diabetes drugs. Almost everyone who takes too much of some of those medications will develop hypoglycemia.

All drugs have potential side effects. Indeed, at some point you’ve probably squinted down the list of possible side effects on the package insert of your medications. They probably include unpleasantries like blurred vision, drowsiness, dizziness, dry mouth, heart palpitations, erectile dysfunction, memory impairment or nervousness. Fortunately, many of those drug side effects occur in only small numbers of people. And if you know how to interpret the list, they may be a useful resource instead of a source of anxiety.

You should suspect a side effect any time you have symptoms not normally associated with your illness or medical condition. Side effects may occur shortly after you start a new medication, after you switch from one brand or formulation of a drug to another, after your doctor raises the dosage or when a new drug is added to your regimen and it interacts with one of your other medications. Though unlikely, you may also suddenly experience a side effect or unusual symptom from a drug you previously had been taking with no ill effects.

Types of drug side effects

Adverse drug reactions may be surprising, but they often happen for a reason. Finding out the cause can determine how to solve the problem.

• Dose-related side effects. This is the result of too much of a good thing. For instance, drugs for high blood pressure may cause dizziness or light-headedness if they lower blood pressure too much. This can happen if the prescribed dosage is too high, if your body is unusually sensitive to the drug or if the blood-pressure drug interacts with another medication that amplifies its effect.

• Gastrointestinal side effects. Problems in the gastrointestinal tract, such as nausea, loss of appetite, constipation, or diarrhea are common side effects. One reason is that drugs taken orally may irritate the sensitive lining of the stomach or intestines.

• Allergic reactions. Immune system responses triggered by a medication can have widespread and occasionally even dangerous effects. Symptoms of an allergic reaction to drugs vary but commonly include skin rashes, hives and itching. Less common but potentially deadly is a severe, whole-body allergic reaction called anaphylaxis. Telltale signs of anaphylaxis include difficulty breathing, rapid pulse and stridor (a whistling or groaning sound while trying to inhale). Anaphylaxis demands emergency medical attention.

• Drug-drug interactions. Drugs sometimes interact in the body, either amplifying or weakening the effect of one or the other, which is why you need to keep your doctor fully informed about every medication or substance you take, including over-the-counter remedies and dietary supplements.

• Drug-food interactions. Some foods and drinks don’t mix well with certain medications. For example, grapefruit contains a substance that affects the activity of an enzyme that processes certain medications in the intestines and liver. Eating grapefruit or drinking grapefruit juice could lead to a dangerous increase in the level of one of these drugs in your blood.

• Drug-alcohol interactions. Alcohol mixes badly with many medications. Among the more dangerous alcohol-drug interactions are those that cause excessive drowsiness or difficulty breathing. It’s wise to avoid alcohol altogether while taking a new medication unless you specifically ask your doctor or pharmacist whether it’s safe.

• Systemic side effects. While some drugs act precisely to target the cause of a disorder, most don’t. Most drugs act throughout the body, or systemically. These drugs affect tissues or systems not involved in the problem being treated. One example is nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin or ibuprofen (Advil, Motrin). NSAIDs reduce pain but can cause an upset stomach and, less commonly, gastrointestinal bleeding.

Drugs that act systemically are more likely to cause side effects than drugs that act locally. One way around this problem is to deliver the drug closer to the organ or tissues that need the help. Generally, locally delivered drugs include topical preparations applied to the skin, eyes, ears, hair or mucous membranes; injections of drugs into the skin or joints; and inhalers that deliver drugs to the lungs.