I can tell you that at the age of 23 I made 7 slashes on each of my wrists in the bathroom, with the door locked, in a bathtub full of warm water and downed a bottle of sleeping pills with whiskey beforehand. It didn't work.
I got sick an hour into it and vomited up some of the pills, but passed out from the blood loss. Unfortunately my parents found me at the time and beat the door down. I awoke in the hospital groggy and hurting from head to toe with my arms bandaged up. And a lecture from an overpaid meat mechanic (doctor). Whom I promptly told to piss off, and climb a stepladder and kiss my ass.
Point is, wrist slashing is far from perfect.