eli5: Why do antidepressants act on receptors or otherwise go through an intermediate step instead of giving you, say, serotonin directly?

Question

I was curious because I was thinking of how many have side effects on your weight or your libido etc. and thought *I wonder if you would have less side effects if they could just give you an extended release dose of neurotransmitters?* I’m sure there’s a reason why you wouldn’t want to add external neurotransmitters and throw off the balance somehow, but I don’t know specifically why.

In: 5

Answer 1

Seratonin doesn’t cross the blood brain barrier, so seratonin taken orally or intravenuously will not reach the brain.

Also these neurotransmitters don’t just do one thing, so it’s not really as simple as just giving you more serotonin. For example, LSD also acts on serotonin receptors but has quite different effects from an SSRI.