So if the great writers of the DCU can’t come up with a plausible Joker toxin, maybe we can. ;)

Let’s start with the effects of JT (which can vary wildly depending on who’s writing the story).

The common symptoms are:

1. Euphoria/uncontrollable laughter, possibly accompanied by hallucinations

2. Seizure (sometimes–again, depending on the writer)

3. Rapid death occurring less than 24 hours after exposure

4. Rictus grin, appearing shortly before death and remaining on the victim’s corpse long after.

[There have been stories where JT also turns the victim's skin white, hair green, etc., but let's stick to the basics.]

Let’s brainstorm!

I’ll start:

For the grin, I like the idea of manipulating calcium channels to force tetany, but I’m not sure it’s possible to target one specific area, like the face. I don’t know, is it possible for a chemical or pathogen to target a specific nerve or plexus? Damage to CN VII could cause perma-grin, could it not?

The biggest problem with the grin, of course, is that you wouldn’t be able to see it unless the corpse was pretty darn fresh. Rigor lasts, what, eight hours? Then every muscle relaxes, regardless of electrolyte balance or present toxin, and you become an expressionless rag doll. If Bats found a body two days dead, he’d have no way of knowing the Joker was behind it without doing some lab work. Am I wrong in that?

Wasn’t that a plot for the Joker fish issues? That he wanted to patent the fish with the joker face?