A side effect is something other than the intended effect. If you used it as a placebo, then it would have a 100% occurrence of lethal side effects, but as it is it has none.
That's true. I'll amend my statement: the law forbids harmful (eg lethal) medicine, not just harmful side effects. A deliberately lethal medicine is simply a poison. The law forbids both killing and suicide, so deliberate poisons are obviously illegal.
I'm not saying this is a good state of things; just that there's no hope of it changing. People will not be allowed to buy euthanasia drugs without a doctor's approval, because it will be seen as legalizing both suicide and poison that could be used to murder someone.
Over at Scott Adams' Blog you can find a very fine example of using the 'Rationality Engine' to solve the social problem of assisted dying.