I need to have a character try and kill themselves, be found latter and survive. Any ideas what drugs he would think would kill him but wouldn't actually?
Homeopathy is an alternative medecine, proven to be no more effective than a placebo. However, not everyone is aware of that and some countries like France use it as a common medication. Your character is in this case: he think that homeopathy are medication, take lot of pills, some alcool, fall asleep due to alcool, but have nothing because they have no active substance
Fun fact: during the 10:23 Campaign, some skeptics performed a "mass suicide stunt" with an overdose of homeopathically diluted snake poison, belladonna and arsenic, in an attempt to publicly show that homeopathy does not work
One major reason is that - at least where I come from - they are injected with a very tiny amount of something that makes you puke. Not noticable when you take a regular amount, but it is going to make you throw up if you take too many.
The purpose of this is to avoid lethal intake of sleeping pills, since they are one of the more common things suicidal people try to overdose on.