This seems like a simple thing to check. 5HTP is a known serotonin precursor, and in most countries is not a controlled substance.
BUT, to anyone reading this with long covid thinking of giving 5HTP a go, for fucks sake PLEASE treat it with respect & do your due diligence with regard to researching the effects and safety of it. Serotonin syndrome is real and deadly. If you are taking an SSRI, or other things that affect / increase serotonin levels then you prob shouldn’t be taking 5HTP.