Drug Side Effects Explained

Medically Reviewed by Nayana Ambardekar, MD on March 16, 2024
4 min read

From the homely aspirin to the most sophisticated prescription medicine on the market, all drugs come with side effects. Many are minor, some are just an inconvenience, a few are serious, and some are just plain strange.

Perhaps the most common set of side effects for drugs that work inside your body involves the gastrointestinal system. Nearly any drug can cause nausea or an upset stomach, though it may only happen to a small number of people. For drugs used on the outside, skin irritation is a common complaint.

To find more about a drug's side effects, look on the label of over-the-counter (OTC) products or on package inserts or printed materials that you get with prescription drugs. Because the inserts often include a long list of possibilities, you may want to also talk to your pharmacist or doctor about what to expect and watch out for.

An allergic reaction could happen with any drug. That can range from itching and rash all the way to a life-threatening anaphylactic reaction.

Some drugs can’t help but trigger side effects because of their chemical structure. The common allergy drug diphenhydramine (also known by the brand name Benadryl) is one. Though it eases allergy symptoms, it also blocks the chemical acetylcholine, and that leads to drowsiness and a host of other side effects, including dry mouth.

Some drugs have barely noticeable side effects at the right dose. Typically, warfarin (Coumadin, Jantoven), used to prevent blood clots, usually works well and isn't bothersome, but serious internal bleeding can happen in the wrong situation.

Side effects may show up only when a drug is mixed with certain other things. This might be called a drug interaction. For example, drinking alcohol while you're taking narcotic painkillers can cause an accidental overdose. This has led to many deaths. Another example is grapefruit juice, which can affect the blood levels of several drugs, including some blood pressure and cholesterol medicines.

Before a drug can come on the market, the FDA must approve it. The New Drug Applications (NDAs) submitted by pharmaceutical companies have, first and foremost, evidence that the drug has the effect it’s supposed to have and is safe. This proof comes from testing the drug, first in animals and then in humans. Once the basic questions of safety and efficacy are settled, the FDA will approve the drug if it deems that its benefits outweigh its risks.

Still, sometimes testing doesn't reveal everything about a drug’s side effects, and they don't show up until after the medication enters the marketplace and more people start using it. That’s where MedWatch comes in. The FDA’s post-marketing surveillance program seeks voluntary input, mainly from health care professionals, on unwanted effects they see in ''the real world.'' Sometimes these reports are numerous or serious enough for the FDA to take regulatory action, such as adding warnings to a drug’s label.

This happened with the psoriasis drug Raptiva. The FDA required that the drug carry the agency's strongest warning, known as a black box warning, after it received reports of brain infections and meningitis in patients taking the drug. The drug was later taken off the market.

The FDA also wants input from consumers when it comes to side effects. All prescription drugs, and many OTC products, must be labeled with a toll-free number the agency has for letting it know about side effects with drugs, called "adverse events." You can report possible new but severe side effects through MedWatch at 1-800-FDA-1088 or through the FDA website

Sometimes, the post-marketing information coming in to the FDA is so disturbing that a drug is discontinued. Baycol, which lowers cholesterol, was strongly linked to a breakdown of muscle tissue that could be deadly. The drug was approved in 1997, and the manufacturer stopped selling it 4 years later. The anti-inflammatory drug Duract spent just 1 year on the market. It was approved only as a  short-term use product, and the FDA found serious liver problems when people took the drug for longer than recommended.

Drug companies are also required to report adverse events to the FDA. Failure to do so can lead to prosecution. In 1985, employees of two drug companies were fined or sentenced to community service for not reporting adverse events involving the blood pressure drug Selacryn and arthritis drug Oraflex. Both products were pulled from the market.

Not every side effect is a bad one. Some are downright welcome.

Take finasteride. Introduced in 1992 to treat noncancerous enlargement of the prostate gland, it was found to regrow hair. Now it's marketed for that purpose under the name Propecia. Today, millions of men use a low dose of finasteride to treat male pattern baldness. Similarly, minoxidil was originally marketed as a pill for high blood pressure and found to grow hair by people who used it. Today, as a cream or foam, it's a popular OTC remedy for baldness.