Thing is they will talk about it anyway if they are that sort. Either as they think you’re lying and using them or are honest and using them, there is no way to stop it, the only way to mitigate it is to not engage at all on it.
as said I don’t discuss my weight at all at work, when someone has commented I simply smile quickly and say thanks and shut it down immediately before it can go further, by changing the subject or moving away , so I’ve never lied, or hidden it, I just don’t engage at all, but I one hundred percent know some will be guessing I used the drugs and gossiping, I lost over 6 stone, it’s hugely noticeable, even I’m surprised by my appearance some days, so assume they will be too, so it’s natural to discuss it.
for friends and family I just was up front, at work, if they are guessing then at least it’s just guess work, they won’t be saying I lied or something, the worst that can really be said is bet she’s used those drugs, and then what, there is nothing more to say, as I haven’t engaged ever on it. I didn’t pretend it was all my own efforts, and I didn’t say how I did it, I simply said nothing.