According to the Colorado Spider Survey, nearly every spider found in the state (or on the planet) is poisonous because they all possess fangs and venoms. That said, some venomous spiders in Colorado can cause particular problems for human beings and those are the ones most people focus on.
Black Widow Spider
The black widow is the most poisonous spider in Colorado and, indeed, all of the United States. These arachnids prefer to live outdoors in masses of spun silk near the ground around rocks and bushes. If found indoors, it will almost always be in a dark, hidden area rarely traveled by humans. Because of their highly toxic venom, a black widow bite is immediately very painful and medical attention should be sought immediately before the venom travels through the body and causes more damage.
While the brown recluse is fairly rare in Colorado, there have been reports of this poisonous spider in the state. Brown recluse spiders prefer warm dark places and are fairly aggressive towards humans. A bite from the recluse is not immediately painful and people often do not seek medical attention until a blister has formed around the bite. Their venom can do extraordinary damage to human cells.
The poisonous hobo spider is a recent addition to Colorado, having moved east from California where they were first found. Often referred to as an "aggressive house spider," the hobo spider has venom that is only slightly less toxic to humans than that of the brown recluse.
These hairy spiders are generally found in the southeastern and southwestern parts of the state. It is rare for a tarantula to bite unless it is provoked. Ironically, while the bite from this poisonous Colorado spider is very painful, it does not usually cause serious damage to humans. Tarantulas also defend themselves by flinging hairs and the hairs can also cause an allergic reaction.