I actually had to decide on this same question a lil while ago with my g/f. I was going to propose to her, and I couldn’t lie to her about me using something and having her find out by finding my gear or walking in on me while I am in the process.
She had an ex who used to shoot heroin and beat her in the past so she is very against anything. So I was to say the least very scared to tell her fearing she would classify them as one of the same.
I sat her down, told her what I was taking, told her what it does, pluses and negatives, and how long I have been taking for and made sure she knew that it was not hurting my body or my mind at all. And that the whole "rage" crap was all bs. She was actually very understanding and just wanted to make sure that I wasn’t hurting myself or the ability to have kids. She basically replied that she is a big fan of anything that is going to get me to look the way I look, lol.
Basically to sum up, if it is someone I truly care about, then yes I would tell them. If it was just some fling or bang buddy, I would keep my mouth shut.