Some mutants, notably, Wolverine have a healing ability, so they basically don't age (or don't age much) and they recover perfectly from wounds and injuries, and they can't be poisoned.
In X-Men Origins: Wolverine, we learn that
Only Wolverine can get the adamantium because his healing ability is strong enough, whereas Sabertooth's is not strong enough.
Granted, this was written after X-Men 2, but why is it that
Lady Deathstrike's healing factor, which must be as strong as Wolverine's since she has adamantium fingernails, doesn't heal the scar from the serum, let alone prevent the serum from working at all?