Tropolisine

Tropolisine is a psychotropic compound known for its hallucinogenic effects. Normally found in certain flowering plants, each tropolisine atom contains a stray neutron. Once it starts to break down in a humanoid's bloodstream it releases an undetectable toxin.

In 2151, while exploring an uncharted M class planet, the crew of Enterprise became affected by tropolisine that was blown down from the mountains during a windstorm. It was discovered by Dr. Phlox that inaprovaline, combined with being removed from the pollen-saturated environment would allow the effects of tropolisine to dissipate. (ENT: "Strange New World")

By the early 23rd century, a permanent cure for the hallucinogenic effects of tropolisine had been found, allowing Humans to settle the planet.

This information was referenced in Archer's biography found in the Starfleet database, which was discovered aboard the USS Defiant in "In a Mirror, Darkly, Part II".