I suspect it comes down to this: we think of suicide as a deliberate act by which a person kills themself. But, what about people who are aware they are playing a dangerous game but no longer care enough (or no longer have the wherewithal to even make an attempt)? At the end of the day, they themselves put the drug into their body. Strictly speaking, they did kill themselves, just in the same way that someone popping a bunch of pills in the medicine cabinet may be trying to do.
Where it gets dicey is if we consider the person in question to be completely “out of control”, at which point it’s common to anthropomorphize the drug and turn it into a kind of killer. But again – drugs must be acted upon to kill.