As Alex has posted on our OMS Forum, the best place to get detailed information about drug risks is from the business section, as these risks dramatically affect share prices.
Hence, he posted this link, which is from Elan itself, outlining the exact risks of developing the frequently fatal condition progressive multifocal leukoencephalopathy (PML) after taking natalizumab (Tysabri).
This is important reading for anyone contemplating starting the drug, or anyone who has been on it for a while. The overall risk of PML for those on Tysabri is 2.5 per 1,000, that is 1 in 400. But this varies with a number of factors. First of all, the data show that the most important factor is JC Virus (JCV) status, that is whether one tests positive or negative for JCV. If one tests negative, then the risk is 1 in 11,000 or less, and doesn't seem to depend on how long one has been on the drug, although the data show that 2-3% of people testing negative convert to positive each year (and around 2.5% of those testing negative have an incorrect result and are actually positive). For those testing positive, the risk of PML depends on how long one has been on the drug, and whether immunosuppressant drugs have been used. For people on Tysabri less than two years, with no immunosuppressant use, PML risk is 1 in 1,786. For those on Tysabri from 2-4 years, with no immunosuppressant use, the risk is 1 in 217. For those who have previously taken immunosuppressants, and on Tysabri less than two years, PML risk is 1 in 625, and on Tysabri from 2-4 years, 1 in 90. Clearly long term use of Tysabri after immunosuppressants in those who test JCV positive, is an unacceptable risk.
For other people considering the drug, it is important to weigh up these risks, and particularly for those with very aggressive MS, balance this against the benefits of a dramatic reduction in relapse rate and progression to disability.