Cartoon characters are famous for being stranded in the desert, finding the nearest cactus and slicing off the top in order to get a drink of water. But the idea of being able to drink clean, non-toxic water from a cactus is as fictional as those cartoons.
Cactus water is highly toxic. The liquid inside a cactus contains alkalis and can induce vomiting if consumed. The fishhook barrel cactus is the only variety of cacti that provides relatively safe drinking water, so it is important to either pack plenty of water before setting out on a hike through the desert or seek other sources of water if stranded.