What D&D game is complete without harvesting dead monsters for their body parts?
The dreaded Troll Serum is one of those sick and twisted ideas I once sprang on my players, which turned out to be hilarious.
First, you kill a Troll, which can be quite tricky if the D.M. Is the kind who plays most regenerating creatures as sneaky, hit and fade predators who use their healing powers to best advantage..
Then you grab an end of it's severed intestine, tie a rock to a rope, ready a bucket, painstakingly thread the rope through the entire length of intestine, and then brutally drag the rock through after it, stripping out the grotesque fluids which line the insides of one of the most voracious eaters in the D&D monster world.
Take the bucket full of green slop and transport it back to your local alchemical lab, then get to work refining and adding the secret herbs and spices.
Once completed, the green and pungent smelling liquid is stored in a large flask, ready to sell to the next customer, I mean, you wouldn't want to try this stuff yourself, not for a second time anyway, and here is why..

Drinking the troll serum requires a high check of the character's Endurance (level appropriate, but it should fairly daunting) as the character slugs back an entire quarter of a gallon (or 1 Liter) of this vicious, acidic, slimy and extremely foul tasting potion.
The effects are not immediate, unless one counts nausea and a stream of constant gagging, retching profanity from the character as an effect.
After a few minutes, the character suffers stomach cramps and an itching sensation, with the first tell tale signs of serum intoxication, eyes that turn vivid green and an increase of +2 to the character's Strength attribute.