Unless ure actually battered out of ure face when they take the blood, the metabolites in ure blood will be just that, inert metabolites, that have no psychological or physiological effect. So you should have no problem, blood tests are quite costly, and you need to specifically look for drugs, by adding a chemical which reacts with the inert metabolites of the drug, causing a colour change or sum other change which can be measured, or summat like that. But I aint sure, cuz you can't donate sperm if ure a drug user. To be on the safe side just ask one of ure teachers or school nurse or whatever, obviously not with illegal drugs, just general questions like 'What if I take an aspirin, or drink coffee, or another painkiller before giving blood? Could that effect somebody they might transfer my blood to, who may have an allergy to one of those?'
Then you'll look like a concerned citizen, and not the infamous drug fiend you are :thumbsup:
And I believe they take a small sample of ure blood thru pricking u with a needle, to see which group it is, and if you're a suitable donor, then they bring out the big needles, find a vane in ure arm, and drain a pint or so out through a transparent tube, then giv you a cookie and sum OJ and tell u to go away.