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

A neurotransmitter only really does something when it gets grabbed by its respective receptor, turning on a signal. So in a sense the SSRI is the direct step and adding new serotonin would be the intermediary step. But how would you add serotonin and have it go where you want? Even MDMA, which is a drug to release more serotonin to your receptors, doesn’t add outside serotonin to your system. There isn’t really an easy way to add serotonin to the brain by ingesting something, and so much of the body (like your gut) is going to react to new serotonin without it easily making its way to the brain.