EDIT: Let’s cool it with the downvotes, dudes. We’re not out to cut funding to your black hole detection chamber or revoke the degrees of chiropractors just because a couple of us don’t believe in it, okay? Chill out, participate with the prompt and continue with having a nice day. I’m sure almost everybody has something to add.
They can’t work immediately because the body isn’t producing enough serotonin to have an immediate effect, nor would you want that. Over time serotonin reuptake is slowed and eventually this has a cognitive effect. That doesn’t help everyone but that doesn’t make them ineffective.