The discovery of a possible fatal side effect typically spells the death of a drug. But the return of two medications previously taken off the shelf for safety concerns represents a notable exception to this rule.

One such drug, Tysabri (natalizumab), had originally received approval from the US Food and Drug Administration (FDA) in 2004 for multiple sclerosis. However, the medication was withdrawn the next year after researchers linked it to a rare and potentially fatal viral infection that affects the brain. (In 2006, Tysabri was reintroduced, under restrictions, for the treatment of multiple sclerosis.) Evidence from recent clinical trials has suggested that Tysabri could ease the symptoms of Crohn's disease, an autoimmune disease that affects the gastric system. In light of these findings, on 14 January 2008, the FDA reapproved the drug for the treatment of this gastric disorder.

Another drug, thalidomide, is also undergoing a revival. In the late 1950s and early 1960s, doctors in Europe, Canada and Japan used it to treat ailments such as morning sickness until they realized that the drug caused horrific birth defects and subsequently banned it. After reviewing new research, the European Medicines Agency recommended it for approval on 24 January to treat multiple myeloma, the second most common cancer of the blood.

According to the FDA, any manufacturer wanting to use a withdrawn or banned drug to treat a disease faces the same obstacles of proving safety and efficacy that any new drug would. But Nora Hansen, director of the Lynn Sage Comprehensive Breast Center in Chicago, notes that it's not easy to convince the medical community that a formerly banned or unapproved drug is acceptable for a new purpose. Fears of litigation over dangerous side effects run high, especially in hospitals.

Still, she adds that “most physicians are open to new uses of drugs with questionable safety records if there is good evidence that it works and safety concerns can be adequately addressed,” such as those regarding pregnancy in women taking thalidomide