In Season 2 episode 14, Reid is kidnapped by an unsub with Dissociative Identity Disorder. The story continues in episode 15 where Reid is injected multiple times with a combination of Dilaudid (drug store heroine) and a psychidelic which causes the flashbacks. At the end of the episode, he shoots the unsub and takes the drugs from his pocket. In episodes 16 and 17, we can see that Reid is suffering from Post Traumatic Stress Disorder as he goes in and out of flashbacks about his kidnapping. He is also seen in the bathroom of a police station pulling the drugs from his bag debating whether to take them before Hotch calls out his name. In episode 18, Reid meets up with an old friend from his childhood, who points out that he can tell that something is not right with Reid, despite that Reid says he is okay. At the end of this episode, Gideon finds Reid in an upscale bar and Reid confesses that he is struggling, but Gideon tells him that "anybody whose been through what you've been through recently, would." This is the the turning point for Reid in which he decides he needs to quit taking drugs, although there are still references to his drug use in other episodes (Season 2 episode 19, Season 2 Finale, Season 3 episode 2, and Season 3 episodes 15 and 16.) Hope this helps.
He gets addicted in the cemetary, being onvulantary shot up and later on more willing. In the end scene you can see him taken the drugs with him, whilst in another episode later he doubts them alone in the bathroom as he struggles his ptss as a means to cope.
I think when Derek mentioned use it to make you a better person/profiler (meaning his empathy but to Reid sounding like an excuse for his drugs to start 'use' to be better and dismiss his ptss complaints)
Than in the following episodes you see his behaviour change till 'fear and loathing he was using, but after the talk in the car he quit and sought help. In an episode in between the team leader discusses a team member having substance abuse problems, but this doesnt mean reid is not jet working on it through the 12 step program (he gives a lot of intell on this group in a much later season and finally does so without looking awkward or getting looks, sure to have beat it )
in episode 'sacred to death' Emily also first asks if Reid is okay, and than again in a different way..i think checkin if Hutch leaving hasn't thrown him back into his into his addiction. there's is an occasional mentioning here and there about it but not directly, this in regard to his fbi status i think, else he would lose his job.
Actually the episodes where he overcomes his addiction don't exist. You see in the episode Elephants Memory he says he quit 10 months ago, if you do the math 10 months before that episode aired was in between season 2 and season 3.