OASC Drugs
Having been just left a recruiting job in an AFCO very recently. I was chatting with the RAF OC in the office over lunch and he mentioned that use of drugs was pretty much a bar to entry in the RAF certainly as aircrew and any experimentation at all was not acceptable. The RN accept that people experiment and it is not a problem if a person has tried a drug once provided it stopped there. I suggest that you don't lie as the Careers officer will have noted it in your report and they will read it at OASC. If it doesn't work out and you are serious about a career as military pilot, try the RN.
Hope it works out for you.