Mescaline is a psychoactive derived from the peyote cactus and others from the Trichcereus family (namely the San Pedro).
The first use of mescaline dates back several centuries, when native American Indians harvested it from the peyote cactus to use in religious ceremonies.
These tribes believed that mescaline provided a link between them and their god, because of the hallucinations and different perspectives it attributes to the user.

San Pedro is the most common cacti that mescaline is harvested from today because the peyote cactus is almost extinct. It is nearing extinction because a single plant can take 100 years to mature.

Mescaline today is cheifly administered orally, although certain preparations allow it to be smoked.
Mainlining is unheard of.

Mescaline is generally accepted to be somewhat a tribal drug and less harsh than other hallucinogenics such as psilocybin (magic mushrooms) or Lysergic Acid (LSD).