When potentially serious side effects of a prescription medication are well documented and become unreasonably dangerous, the drug may eventually be removed from the marketplace by the U.S. Food and Drug Administration. Usually, when the FDA believes it is clear that a drug no longer has a place in treatment, it will ask the manufacturer to withdraw the drug voluntarily. If a company does not agree, the FDA can bring formal proceedings to require withdrawal.

Many complex factors go into deciding whether a drug should be taken off the market. Here are some major issues, often overlapping, that weigh into the decision-making process.

Rare, Unpredictable Problems

Most drugs on the market are well-tolerated, and their adverse effects are known. Known side effects cause more injuries and deaths than unrecognized side effects. But some problems happen so infrequently that they can't be seen or predicted before a drug gets on the market. Drugs are typically tested in several thousand subjects, allowing for the detection of relatively common, serious adverse events, such as those affecting 1 in 1,000 people. The practical size of clinical trials means we can't know everything about a drug when it gets on the market. Rare events will only surface when the drug is used in larger numbers of people. Sometimes less severe events that are seen in trials can be used to predict the occurrence of rare, more serious events, but that is not always the case, and such predictions have considerable uncertainty.

More Toxic than Expected

There are also times when a drug's toxicity is known, but the drug turns out to be more toxic than the clinical trials suggested, which again may only be seen when the drug is used in larger numbers or in different ways.

Initially approved in 1997, Baycol (cerivastatin) was a member of a class of cholesterol lowering drugs known as "statins." Baycol and the other five drugs in its class--Lipitor (atorvastatin), Lescol (fluvastatin), Mevacor (lovastatin), Pravachol (pravastatin), and Zocor (simvastatin) -- have all been associated with rare reports of rhabdomyolysis, a condition that causes marked breakdown of muscle cells and can sometimes lead to fatal kidney failure and other problems.