The film "memento". WTF?

This is an excellent film about a guy called Leonard, who can’t form new memories following an attack on his house in which his wife was raped and he suffered violent head trauma.

It is shot in reverse, you see the ending first and move gradually back in time through a set of overlapping flashbacks in which Leonard has to deal with meeting the protagonists anew in every scene. I heartily recommend it to anyone who hasn’t seen it, apart from a superb set of performances from the leads, it raises intersting questions about perception of reality, trust, and whether one can really know anything.

WARNING, SPOILER AHEAD

BUT, for anyone who has seen it, what’s the deal with Sammy? The plot speeded up so quick towards the end that i missed out on whether Sammy really overdosed his wife, or was it Leonard’s wife who induced Leonard to overdose her, and if so how come Leonard (whose inability to construct new memory presumably dates from his assault in “the incident”) managed to build this fantasy about Sammy’s wife?
Presumably Sammy really existed, but can we be sure, or is the whole thing a lie from “Teddy” who wishes to exploit Leonard just as Natalie does?
Man, this one makes fight club look simple.