Really...simply: it's doesn't matter. IF (and that's a huge if) law enforcement were to take interest in identifying you (extremely unlikely--unless you talk about manufacturing or commerce in scheduled drugs) AND if they chose to try to prosecute you (for USING drugs at some time in the past, based on your admissions here) they could do so equally well whether you use "I" or some other term that can be shown was obviously intended to imply yourself. Not using "I" wouldn't stop any prosecutor--and would be accepted in any court.

A better thing to do, if you wish to avoid any possible trouble, is to contact the site ONLY through the tor network, or an overseas VPN (or both). If you can't be found, you can't be prosecuted. If you COULD be found the world would have to change RADICALLY (like, sci-fi/dystopia radically) before it would be likely any jurisdiction would be interested in prosecuting you for saying in a forum that you have had drug experiences. And if they DID want to do such a thing--avoiding use of "I" wouldn't stop a successful prosecution...

OTOH...a disclaimer signature saying that you're pretending about what you post MIGHT be of some benefit in some courts, IMO. Still, the best protection is IP protection.

Truth is...it's almost certain there's nothing to worry about (unless, of course, you one day hope to work for the FBI/CIA, etc., and would need to pass an ultra-invasive security exam investigation...in which case...you'd probably fail the polygraph exam, anyhow).