I think there is a segment of the population that hears about the use of weight loss drugs and they get anxiety for the person who is taking them. Sort of, 'hey, good for you, but I'm really scared about you dying from the drug in the future' going on in the back of their head.

Also, when people don't have self control to manage their weight, some people will judge them as having low will power, so the admission that someone had to use a weight loss drug can cause others to judge them as less-than-themselves because they needed to use a drug to get results.