For people who have mentioned Tylenol poisoning, here's what it does:
It destroys the liver, resulting in a long and painful death over the course of about 3 weeks.
The mere thought of that is enough to make most people cringe. Oh hell no. And fact of the matter is, most common household pills do something similar; an OD almost invariably just destroys the liver, resulting in a horrible delayed death. The whole "Take a bunch of pills, fall asleep, and die" is largely a myth. It takes very specific prescription drugs to do that.
Now there ARE other things that cause that basic effect, but I'm not going to discuss that simply because giving a suicidal person information on how to painlessly end their life is a surefire way to make it happen. What stops a lot of people is the thought of the pain, or the difficulty involved with carrying out a particular method. In many cases if you were to give them an easy, painless way, they would take it and you would lose them.
Because of this, I will never, EVER give someone ideas. This gives you further ammunition to save someone when they say they are planning to use a particular method, because then you can tell them what a horribly painful or ineffective method it is, and thereby cause them to scrap their plans because they don't know what else to do. Don't laugh. I've done it. Knowing how horribly BAD some methods are can be used to stop a potential suicide. Never, ever mention anything GOOD about any particular method to a potential suicidee.
I've walked both paths. I've walked the path of the severely depressed and even suicidal, and the path of one trying to stop a friend from taking his own life. I've learned the hard way how the depressed mind works. Suicide comes out of a need to stop the pain, shame, or guilt. Sometimes the only way to stop someone is to emphasize that taking their own life by a particular method would only add more to what they are already suffering. Since they are already at the point where they cannot take more, it will usually cause them to shy away. Therefore you need to keep that card, and play it well. And whatever you do, don't give them any more ideas.