Assuming the drug adds another 20 years of quality life, the impact on social program financing would be very good. As is, the average person spends, roughly ~56% of human life being net-productive (20 learning, 45 working, 15 retired). Add 20 healthy years, and most folks will keep working, which would raise the share of productive years to ~65%. If the treatment is expensive, then only net-productive individuals would be able to afford it, meaning we would gain the benefit of contributors contributing for longer, without net-recipients receiving for longer.
This would also allow folks another 20 years of planning/saving for retirement, which could greatly improve the quality of those years for the average retiree, but probably won't.
The biggest social impact of this drug, I think, would be its affect of the wealth gap. Another 20 years of compounding market returns would buoy the net worth of the 30% who make up the "upper class". An ever-increasing number of people would be able to achieve financial "escape velocity" by building nest-eggs which can sustain them indefinitely via 3% withdrawals, allowing them to retire "early" (i.e. before the new "standard" retirement age of 80). Folks who speed-run their careers and retire in their 40s-50s could enjoy 50 or 60 years of retirement! Inheritances would rise dramatically, but come later in life. This would also result in a backlash, of course. Envy is the oldest and most powerful organizing "principle", and politicians will happily use this new prosperity to generate enmity for their own purposes, just as they always have.
My main concern would be the dysgenic impact of such a thing. If it becomes generally available, it will no doubt encourage thinking folks delay having children even more, while also granting another decade of fertility to the "oops another baby" cohort. It seems clear that this would significantly accelerate our decline toward idiocracy.