The true prevalence of PSSD has yet to be determined, although published calls have been made for post-marketing epidemiological studies.

While the panel recommended approving the drug, it said post-marketing studies should be conducted to determine any adverse long-term effects.

But other post-marketing studies may yield far less data.

A post-marketing study, conducted by the innovator, in over 12,000 patients showed that only 0.75% of them developed adverse events attributable to the drug.

In a post-marketing study conducted in 1500 Indian patients, only 9 adverse events were reported in 9 patients (0.63%) that were of mild to moderate intensity.

Phase IV clinical trials, also called post-marketing studies, are conducted after a treatment has been approved.

In the United States, the Food and Drug Administration (FDA) is responsible for monitoring post-marketing studies.

It wants the agency to be given explicit power to compel post-marketing studies and to impose fines, injunctions and withdrawals to enforce its decisions.

Unfortunately, it has a poor track record in forcing companies to complete post-marketing studies.

One report of PML in a Raptiva-treated patient came from an ongoing post-marketing epidemiological study of patients with psoriasis.