Good question. Maybe only certain people have access. Can't have the interns floating around stoned.

@tonyismad said:

Sam is just gonna cut Jorah Mormont in cold blood? With just rum to dull the pain? In other episodes the use milk of the poppy (opium) for pain. Why not this time? I'm sure they have some at The Citadel.

Exactly what I thought. I guess he couldn't take any without being noticed so he had to use a substitute, but that does not explain how he got all of his surgical tools and ingredients for the operation.

I'm sure poppy is well guarded and would have gone noticed.