Never turn down a drink from an NPC.

This actually goes double if you suspect it's poisoned. In that case, accept graciously, and then dump the drink into a nearby pot plant* when they are not looking. Because refusing would be bound to be worse.

* For American English speakers, this means a plant in a pot, not what you were probably thinking of. Although that would do fine in a pinch.

Transcript

R2-D2: So, here we are again.
Luke: {over comlink} R2, come in. Are you sure this is the way to The Tree? I really don't want to run into Yoda.
Yoda: {over Luke's comlink} Run into him, you have! Hmmm?!
Luke: {over comlink} Oh, hi. I was... er... just looking for you.
Yoda: {over Luke's comlink} Found me, you have! To my hut come, and a glass of swamp juice, have.
R2-D2: Right, I'll stay here until you get back. Good luck with the swamp juice.
Luke: {over comlink} Okay. Fine. Just keep your eye on the little details this time.
Luke: {over comlink} Like don't let the ship sink into the swamp.