Basically not much can happen when dealing with mescaline cacti. At worst, you sting yourself with a thorn or get caught by the police. Dealing with the cactus itself not emanates many significant hazards. Only three.
It is described in literature, that strong overdoses of mescaline can cause respiratory paralysis. Practically, however, such a case has never been documented. The lethal dose for a mouse amounts to 880 milligrams per kilogram/body weight. Applied to humans, this would mean an adult would theoretically have to take up approximately 70 grams of mescaline to drop dead.
Inexperienced psychonauts could confuse Mescaline cacti with plants of the spurge family. Particularly Peyote is quickly confused with the toxic Euphorbia obesa - which should not be eaten, because this is where the fun ends. Therefore: experience, knowledge and knowledge of the facts are the best bodyguards.
Like all psychedelics, mescaline can naturally activate a latent psychosis or lead to a bad trip. In the case of sudden psychosis paramedics will administer a benzodiazepine, such as Valium, Ativan or Versed. These drugs usually stop a psychedelic trip within minutes. In case of a bad trip the user should be calmed by talk down, supplied with water and fresh air and lovingly cared for by friends.