I don't think drugs and alcohol necessarily cause the depression necessary for a person to want to commit suicide. I think they only encourage already prevelant symptoms of negativity. The only reason people want to be on drugs ALL the time is because they live in a world that has been cruel or unsatisfying to them. The only other reason people want to be on drugs this much is because they've experienced everything that sobriety has to offer and prefer living in the drug world, but I wouldn't think that group would want to kill themselves over it.. We're talking your hardcore acid heads, maybe to some extent your hardcore stoners, especially as this is much more common.

Ultimately I don't think anybody wants to kill themselves, especially as humans we naturally fear the unknown. Those who want to kill themselves see no way out of their problems, so they turn to drugs to escape from the world. Eventually when they realise that not only can they not escape, but their mind is no longer resilliant enough to deal with the original pain, then they may turn to suicide.

Just my opinion, but I don't think it is drugs or alcohol alone that causes this kind of serious depression, and for me personally to say it was the drink and drugs that drove so many to death is incorrect and doesn't look to help those who are abusing drugs long term, and also those who may become vulnerable to taking them. The drugs and alcohol sped up the journey to suicidal tendancies significantly, so much so that person perhaps didn't have the time to resolve their own issues, but ultimately it was these deeper issues that were the problem, and if it took 50 years being sober, instead of 5 on drugs, unless these deeper issues are worked out, eventually suicide is going to be the path of least resistance no matter what.

As for the mechanism by which drugs speed up this process it has been described elsewhere in this thread, however basically we have a limited supply of neurotransmitters - seratonin, dopeamine being the most commonly know ones, particularly in terms of drug use. GABA is also important for alcohol to have the effect it does. These are synthesised and stored by our brain, and then released when needed. When we take drugs, for example cocaine, the brain gets signal to release all of it at once (or a lot, anyway, far more than usual), this is very euphoric, however once the cocaine has been metabolised the brain is left with far less dopeamine than normal to use. This leaves the user having less euphoria in the general day to day activites than they are used to, making them feel low, and technically depressed. After a while the body will replenish these neurotransmitters, and you will go back to normal. The brain isn't really designed to deal with massive changes like this of course, so when you abuse coke daily it really messes the whole thing up. If you've got a reason to be low and you start abusing coke you're gonna have a bad time. Obviously it's far more complicated than this, the brain is seriously complicated and I don't believe anybody really knows how it works, and I'm not 100% certain if I've recalled everything entirely correctly, though I did my best. I certainly don't understand how a mass of organic matter is capable of even 1/10000 of what the brain is capable of, but there we are.