system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why are some hormones such as melatonin available for sale OTC, while dopamine, serotonin, etc. isn't?

I know nootropics may help increase these levels, but dopamine is both a neurotransmitter and a hormone, so why do I have to rely on Reddit to get it?
gpt
Dopamine and serotonin don't cross the blood brain barrier, so ingesting them won't do anything. They do things outside of the brain though, and it turns out those things are pretty much always bad if no carefully and properly dosed. It's the same reason why you can't buy many other drugs OTC. Melatonin on the other hand is pretty benign. It's pretty much impossible to overdose. If you take too much, you'll just be excessively tired, irritable, and have a headache.