No one who brings up Fen Phen seems to grasp how long both that and GLP-1s have been on the market. We're up to 4x Fen Phen's run already (5-years Vs. 20-years). GLP-1 Agonists aren't new, they've just been approved for additional usages.

So why, after 20-years, and millions of people haven't fen-phen-like side effects appeared?

That was one of my first thoughts.

It’s perfectly possible for a new hot to have a severe side effect that won’t be noticed for quite a long time.

Semiglutide appears to have undergone final clinical trials in the US around 2017. Given it hasn’t been on the market terribly long and has only an exploded in popularity relatively recently it doesn’t seem like it would be that hard for it to have a serious side effect in a small portion of the population that hadn’t been detected before due to the limited number of people taking it, the amount of time it takes to manifest, or both.

Obviously it’s providing significant benefit that risk could easily be worth it. But as it gets marketed towards more and more people that won’t be true for all of them.