THC is one of many cannabinoids found in the cannabis plant. Most of the THC is concentrated in stalked capitate trichomes. Trichomes are glandular hairs found on many plants and are found in several types on the plant. It is likely that such hairs deter predators by irritating the mouths of herbivores that would attempt to eat the plant. Glandular hairs like cannabis trichomes may also help the plant in cold or windy conditions, protecting the plant surfaces from frost and drying out. When a glandular trichome is ruptured, its sticky resins leak out, serving to capture tiny insect predators. The psychoactive element of the glands may also deter predators. Many of the cannabinoids contained in cannabis are antibiotic, protecting the plants from disease. The stickiness of these resinous glands help attract and retain pollen thus aiding in the plant's reproduction. Unpollinated female plants produce more and more trichomes in an effort to catch pollen, thus producing more THC than pollinated seed bearing plants. So while not a "poison" per se, THC and other cannabinoids act as predator and bacteriological deterrents, offer protection from the elements, and aid in the reproduction of the plant.