People don't overdose because they buy too many drugs. They overdose because they combine drugs they shouldn't (like heroin with barbiturates) or because they buy drugs that have been cut with lethal substances, or because a batch is too strong and although they take the same dose they take usually, they end up taking too much. Or even because they're used to taking cut heroin and they suddendly find some that is unusually pure and take their regular dose and overdose.