There's obviously potential downside, but for people with serious illnesses there's potential upside.

For example: If you've got a disease that's going to kill you and unlikely helped by current medication/therapies, then dying a bit early is a small downside, not dying for however many more decades, on the other hand, is a huge upside.

In this case, the upside/downside is asymmetrical in favour of the upside! It would be logical to take 'under-tested, possibly lethal, drugs'.