They're not necessarily too stupid. The clever drug salesman, a.k.a. psychiatrist, just puts their fears aside.
"Oh," he'll say, "That only happens in less that one per cent of cases, and if it does, we'll be monitoring for it and it's completely reversible."
"Ah," he'll swindle, "But there's no other way. This is my honest recommendation, you can't deal with these problems without medication."
Or my personal favourite, because I do have friends who have been thusly cheated, "Oh, don't worry, it's just a problem with the dosage, we can fix that my making me more mon- *cough* I mean, giving you a little bit higher dose. I assure you, this will work, just give us time to figure out the dosage."
So sometimes not stupid (but yes, some people just don't care about their children) - but cheated and swindled.