[adrianN]

The obvious upper bound on "forever" is the useful life of the universe (i.e. enough negentropy is left to support life). More realistically, no pill will prevent your body from being horribly mangled in an accident and very low risks for accidents pile up to probability 1 over long enough times.

[dennisgorelik]

"Accidents" could be solvable to -- by backups. You would be able to restore your body from backup.

[TheCoelacanth]

Even then, there will be a small chance of something happening that kills you and destroys all of your backups. Over a long enough time span, the probability of that happening approaches 100%. You could make it a very long time by doing that, but not forever.