The thats in the sentence above is non-standard English. The correct word (in standard English) would be whose, which indicates that the noun which follows it belongs to or is associated with the person or thing mentioned in the previous clause (in this case a thing: a poisonous substance). I don't support people's usage of a poisonous substance whose high is not making you feel like sh!t when you're not smoking it. The sentence is still unclear because of the 'is not making you' part, but the thats problem has been fixed (whose (and thats in the non-standard version) is a relative possessive determiner).
Without reading this unnecessarily long thread: "that's' is a contraction for "that is". If "that is" isn't what you mean (if it doesn't make sense in the sentence when you say it that way), then skip the apostrophe. My best advice. - PV
