So, you have a large, but not too large (bigger than a horse, smaller than an elephant) dragon, who's too agile and smart to be hunted with ballistas. So, dragon hunters decide to use crossbows. The bolt is coated with a special poison, that should weaken the creature to the point of near death. The bolt itself can't penetrate deep enough to cause harm, but it should be able to reach the bloodstream.
The poison of choice is atropine. Medication in small dosages, poison in high dosages. Here's the list of symptoms:
A person with atropine overdose is hot as a hare, blind as a bat, dry as a bone, red as a beet, and mad as a hatter.
The problem is you probably want to recover the dragon's body. Reasons vary from valuable resources to showing that the dragon has indeed been slain. So, if the creature chooses to fly away, the poison should work quickly to bring the dragon down before they get too far.
How could the speed and adverse effects of the poison be increased without adding extra mass to it?