Only if it can be proved with 100% certainty that they knowingly took it and it did improve their performance. Recreational drug use should still lead to a ban though.

I think that they should probably be threatened with the possibility of a lifetime ban after being caught a second time using drugs that help an athlete's performance - cheating is cheating, and should be discouraged. But I don't know why 'recreational drugs' use should make any difference? After all, a pop singer isn't "banned" from being a pop singer.....ditto film stars and the like. Whats the difference?
If the drug used hasn't been used with the intention of improving performance as an athlete, it's immaterial, I think.