I've dabbled with heroin and have seriously considered it as an option for ctb, although most likely paired with another method, probably hanging. If you're not averse to needles and could successfully get a full syringe into your veins, I think it would be a pretty solid way to go.
I had a close call once when I shot for the second time. Got the stuff in and felt like I'd nodded off for just a second, but the people with me told me it had been well over an hour. They told me I was swallowing my tongue and choking, so they might have saved me that time. It was this really wonderful rush until I lost consciousness, then I felt like rubbish for the next two days.
If I chose this method, I'd rig up a noose above a stool or something then shoot it. I'd guess that by the time you'd lost consciousness enough to completely lose balance and topple over there'd be enough of the drug in your brain that you wouldn't feel a thing.
