Huachuma, or San Pedro, is a psychedelic cactus in the genus Echinopsis native to the mid-elevation Andes. Shamanic practitioners across huachuma’s wide range have prepared the plants as a visionary drink for at least 3,000 years. Scientific and public interest in huachuma for its psychedelic effects is quickly increasing, but there is a noted lack of phytochemical information about the plants and their natural diversity. This study compares 5 geographically diverse San Pedro cacti from Peru and their resulting medicinal preparations, both phytochemically and in the qualitative experience of a Peruvian huachumero (San Pedro shaman). We employ a field sampling technique called solid phase microextraction (SPME) to capture quantitative data and metabolic fingerprints of the plants both raw and after preparation as medicine. SPME has been used successfully in previous phytochemical studies and is particularly fast and inexpensive when collecting large numbers of samples in the field. LC-MS/MS was used to identify the major alkaloids mescaline and hordenine, and to discover how they vary quantitatively between diverse plants. For the qualitative portion of the study, the Peruvian huachumero who prepared the medicine drank each preparation separately and reflected verbally on the plants’ characters and personalities. Results of the phytochemical study showed very similar metabolic fingerprints and little difference in alkaloid quantities between plants, but the qualitative experience of ingesting each of the plants was unique. This supports anecdotal evidence from traditional San Pedro use in Peru, which suggests that indigenous communities and practitioners distinguish types of San Pedro which science cannot tell apart. These results also show the utility of SPME as a field technique for psychedelic phytochemists.