I've never had it either and I have dabbled in substances (weed anyway - and I did try a few other things as a teen but only once or twice out of curiosity). I just told the doctor or psychiatrist or whoever was asking the question the truth if they ever asked. It's not something I feel entirely ashamed of, or at least not too ashamed about to be able to talk about it in the right company, as it all felt necessary at the time. There's things I'm more ashamed of.

If you're clean anyway then I don't suppose it matters. If the trust thing bothers you then just assume this pdoc has a system of screening everyone as a matter of course rather than selectively picking you out through lack of trust. I notice a lot of companies are starting to do that in the job market. Good thing my old work never or they'd have no staff.