I am reading the old AD&D campaign guide to Waterdeep (TSR 1109 - City of Splendors) which has a chapter with a list of the written laws of its Code Legal. I did not find any laws against the sale of poisons and drugs (poisoning of city wells is expressly sanctioned) but I read in several sourcebooks this kind of trade takes place almost entirely in Skullport. What is the reason for this? What does happen if a drug dealer is cought by the guards inside the city's walls?
Forced to pay taxes for running an apothecary or GTHO? Oh, right, Waterdeep. Then it's like with smokepowder, probably.
People never wonder How the world goes round -Helloween And even I make no pretense Of having more than common sense -R.W.Wood It's not good, Eric. It's a gazebo. -Ed Whitchurch
What I've found about Waterdeep and poisons (and any potentially dangerous substance/item, really) is that such substances seem to be quite legal until something public happens involving them.
The second Vampires of Waterdeep adventure mentions a yuan-ti narcotic/poison that was only made illegal when a nobleman and his retainers were assassinated by it. Before then, it was easy to ship in and out of the city, but afterwards it had to be smuggled.
It seems to be a mentality that has survived the times. Smokepowder was fine until after residences started exploding and Dream Spheres were all the rage until they started incapacitating large numbers of people.
When life turns it's back on you...sneak attack for extra damage.