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 taken internally involves the gastrointestinal system. Nearly any drug can cause nausea or an upset stomach, though it may only happen to a handful of users. For drugs used externally, skin irritation is a common complaint.

Drug Side Effects: Types

Side effects fall into several categories:

Allergic reactions can happen with any drug and can range from itching and rash all the way up to a life-threatening anaphylactic reaction.

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

Some drugs typically have barely noticeable side effects when dosed properly. For example, Warfarin (Coumadin, Jantoven), used to prevent blood clots, is usually well tolerated, but serious internal bleeding can occur.

Side effects may only occur when certain drugs are mixed with certain other things. These might also be considered drug interactions. Drinking alcohol with narcotic painkillers has caused an alarming increase in accidental overdose deaths. Drinking grapefruit juice can affect the blood levels of several drugs, including some blood pressure and cholesterol medicines.

To find more about a drug's side effects, information about them is available on the label of over-the-counter drug products and on package inserts or printed materials dispensed with prescription drugs. Because the inserts include such a long list of possible bad effects, it is very helpful to also talk to your pharmacist or doctor if you have any questions regarding a drug's side effects.

Drug Side Effects: The FDA's Role

Before a drug can come on the market it must be approved by the FDA. The New Drug Applications (NDAs) submitted by pharmaceutical companies contain, first and foremost, clinical evidence that the drug has the therapeutic effect it’s supposed to have and is safe. This proof comes from testing of 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 not everything is known about a drug’s side effects until after it 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 adverse effects they may be seeing in ''the real world.'' Sometimes these reports are numerous and/or serious enough for the FDA to take regulatory action, such as adding warnings to a drug’s label. One example of that involves 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 withdrawn from 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 maintained by the agency for the purpose of reporting side effects with drugs, called "adverse events." You can report possible new but severe side-effects through calling MedWatch at 1-800-FDA-1088 or through the FDA web site: www.­fda.­gov/­Safety/­MedWatch/­HowToReport/­default.­htm.

Sometimes, the post-marketing information coming in to the FDA is so disturbing that it results in a drug coming off the market. Baycol, which lowers cholesterol, was strongly linked to a potentially fatal breakdown of muscle tissue. Approved in 1997, it was voluntarily withdrawn four years later. The anti-inflammatory drug Duract spent just one year on the market. Approved as a strictly short-term use product, the FDA found serious liver problems with people taking the drug for longer than what was recommended.

Drug companies are also required to report adverse events to the FDA, and failure to do so can lead to prosecution. In 1985, employees of two drug companies were fined and/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.

Drug Side Effects: Surprising Results

But 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 (and is 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, originally marketed as an oral tablet for high blood pressure, was found to grow hair in those using it. Today, as a topical lotion or foam, it is a popular over-the-counter remedy for baldness.

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