This is a myth ! Or more exactly, confusion. Mescaline* is the psychoactive component of the peyote cactus. This cactus is unrelated to agave, but the words are close hence the confusion.
*Mescaline is a hallucinogen obtained from the a small, spineless cactus Peyote (Lophophora williamsi). Mescaline is also found in certain members of the Fabaceae (bean family). From earliest recorded time, peyote has been used by natives in northern Mexico and the southwestern United States as a part of traditional religious rites.
The top of the cactus above ground, also referred to as the crown, consists of disc-shaped buttons that are cut from the roots and dried. These buttons are generally chewed or soaked in water to produce an intoxicating liquid. The hallucinogenic dose for mescaline is about 0.3 to 0.5 grams (equivalent to about 5 grams of dried peyote) and lasts about 12 hours. While mescaline produced rich visual hallucinations which were important to the native peyote cults, the full spectrum of effects served as a chemically induced model of mental illness.
Mescaline is used primarily as a recreational drug and is also used to supplement various types of meditation and psychedelic therapy.
Effects of mescaline
Users typically experience visual hallucinations and radically altered states of consciousness, often experienced as pleasurable and illuminating but occasionally is accompanied by feelings of anxiety or revulsion. Other effects include: open and closed eye visualizations, euphoria, dream-like state, laughter and a psychedelic experienceSide effects of mescaline
Side effects of mescaline use may include: anxiety, racing heart beat (tachycardia), dizziness, diarrhea, vomiting and headache. Like most psychedelic hallucinogens, mescaline is not physically addictive. Mescaline containing cacti can induce severe vomiting and nausea, which adds an important part to traditional Native-American or Shaman ceremonies as it is considered cleansing.
SOURCE : https://www.drugs.com/illicit/mescaline.html