For starters, in addition to providing super-strength and limited invulnerability, it also gives the user limited protection against being detected by Seers. This is helpful in that it protects the makers of the drug from being "ratted out" through the telepaths.
Second, while it physically resembles Superadine, extended use of Fixadine does not transform people into either Trolls or "Supa Trolls".
Third, withdrawal from Fixadine does result in the gradual shutdown of one's internal organs until the person dies in agony.
And fourth, scientists in the Primal Universe speculate that Fixadine could actually treat addicts of Superadine, but they still have to get around the whole agonizing death part.
(See its Paragon Wiki entry for more information.)