The description below also applies for Trichocereus Peruvianus.

The San Pedro cactus has gained considerable fame in the past five years after numerous reports that it is hallucinogenic, Contains mescaline, and is readily available from cactus nurseries. This plant known botanically as Trichocereus Pachanoi is native to the Andes of Peru and Ecuador. Unlike the small peyote cactus, San Pedro is large and multi-branched. In its natural environment, it often grows to heights of 10 or 15 feet. Its mescaline content is less than that of peyote (0.3 - 1.2 percent), but because of its great size and rapid growth, it may provide a more economical source of mescaline than peyote. One plant may easily yield several pounds of pure mescaline upon extraction.

San Pedro also contains tyramine, hordenine, 3-methoxytyramine, anhalaninine, anhalonidine, 3,4-dimethoxyphenethylamine, 3,4-dimethoxy-4-hydroxy-B-phenethylamine, and 3,5-dimethoxy-4-hydroxy-B-phenethylamine. Some of these are known sympathomimetics. Others have no apparent effects when ingested by themselves. It is possible, however, that in combination with the mescaline and other active compounds they may have a synergistic influence upon one another and subtly alter the qualitative aspects of the experience.

The effects of San Pedro are in many ways more pleasant than those of peyote. To begin with, its taste is only slightly bitter and the initial nausea is not as likely to occur. When the full psychotropic experience takes hold it is less overwhelming, more tranquil than and not nearly as physical as that from peyote.

Preparation: San Pedro may be eaten fresh or dried and taken in any of the manners described for peyote. Cuttings of San Pedro sold in the USA are usually about three feet long by four inches diameter. A piece 4-8 inches long will usually bring about the desired effect. The skin and spines must be removed. The skin should not be thrown away, however. The green tissue close to the skin contains a high concentration of mescaline. Some people chew the skin until all the juices are extracted. If you don’t what to do this, the skins can be boiled in water for several hours to make a potent tea. The woody core of the cactus cannot be eaten. One can eat around it like a corn cob. The core does not have much alkaloid content, but can be mashed and boiled as a tea for what little is there.
Note on Trichocereus Peruvianus.
The Trichocereus Peruvianus is more potent than San Pedro. A typical dose (equivalent to 200-400 milligram of mescaline) requires about 300-500 grams of fresh plant material (30-50 grams dry).