What San Pedro is

A mature San Pedro is a cluster of upright stems, normally 3 to 6 metres tall in habitat, each 6 to 15 cm across. The stems are light to dark green with usually 6 to 8 ribs and whitish areoles, the small cushions the spines grow from. The flowers are white, strongly scented, around 19 to 24 cm long, open at night and last about two days. These figures come from the species account on Wikipedia, which cites the botanical literature.

In cultivation the plant can add around 30 cm of growth in a good year, fast for a cactus. It is easy to propagate from cuttings, and one widely grown clone is known among growers simply as the Predominant Cultivar, or PC.

Three cacti are sold under the San Pedro banner. They look alike at first sight, and even specialists disagree about where one ends and the next begins.

The Bolivian torch also exists as a monstrose form with short, irregular, branching segments, sold under the nickname "penis cactus". The descriptions in the table follow the Wikipedia accounts of E. macrogona and E. lageniformis.

The naming is untidy. Botanists have moved these plants between Trichocereus and Echinopsis more than once, and the borders between species are still argued over. A Peruvian government resolution from 2022 lists pachanoi, peruvianus, cuzcoensis, bridgesii and terscheckii as the main kinds that people in Peru call San Pedro (El Peruano, 17 November 2022). For a buyer the practical point is simple: a shop label tells you roughly what you have, not exactly.

In the Andes the cactus has many names. The best known are huachuma, wachuma and achuma; the Peruvian resolution notes that achuma comes from Aymara, which suggests the use once reached much further south than northern Peru. Other names recorded there include hierba santa, cardo santo and huando hermoso. Cimora appears in the ethnographic literature both as a name for the cactus and for the bitter drink made from it in northern Peru. San Pedro, Saint Peter, is a Christian name from the colonial period. More on this on the history page.

The psychoactive compound is mescaline (3,4,5-trimethoxyphenethylamine), the same alkaloid found in the Mexican peyote cactus (Lophophora williamsii). Mescaline acts mainly on serotonin 5-HT2A receptors, like psilocybin and LSD, and also binds to some noradrenaline and dopamine receptors (Vamvakopoulou et al. 2023). San Pedro also contains smaller amounts of related compounds such as hordenine and tyramine.

The most cited measurements, from a team at Sul Ross State University in Texas, were made on the green tissue just under the skin of the stem.

Ogunbodede et al. (2010) measured 14 taxa and cultivars of the San Pedro group with one consistent method. Every sample contained mescaline, but the amounts ranged from 0.053% to 4.7% of dry weight, a factor of almost 100. The highest values were found in E. pachanoi from Peru, the plants most associated with traditional use.

Three things follow from that study:

• The species name on a label does not tell you the strength. Two plants of the same species can differ many times over.
• Fresh cactus is mostly water, so a fresh weight says even less than a dry weight.
• Nobody can tell the content of a plant by looking at it. Thickness, colour and spines are not reliable guides.

This is the main reason we do not give gram amounts of cactus anywhere on this site. The safety page explains what the variation means in practice, and the effects page describes what mescaline does at known doses of the pure compound.

Peyote (Lophophora williamsii) is a small, low, button-shaped cactus from Mexico and southern Texas. San Pedro is a tall, fast-growing columnar cactus from the Andes. Both contain mescaline. Legally they are often handled differently: the United States and France name peyote explicitly in their drug schedules, while neither names San Pedro. The legal page has the details per country.