After a four-year stay in South America Mantegazza published this report on medical observations on the use of Erythroxylon coca leaves of the populations in the places where he stayed and practiced. He reported that, as a result of the drug, natives who constantly chewed a bolus of coca leaves and ingested their juice, exhibited great energy and resistance to hunger, cold, humidity, bad weather and hard work, even in places of high altitude.

Mantegazza also reported that coca leaves, taken as an infusion or chewed, recovered from the most varied gastrointestinal affections. He also reported cases of abuse and the onset of addiction to coca, and the results of the experiments he carried out on himself with the ingestion of increasing quantities of the juice of the chewed coca leaves.

The Wikipedia article on Mantegazza quoted his expression of the high experienced as a result of the drug:

"... I sneered at the poor mortals condemned to live in this valley of tears while I, carried on the wings of two leaves of coca, went flying through the spaces of 77,438 words, each more splendid than the one before...An hour later, I was sufficiently calm to write these words in a steady hand: God is unjust because he made man incapable of sustaining the effect of coca all life long. I would rather have a life span of ten years with coca than one of 10 000 000 000 000 000 000 000 centuries without coca."

Sigmund Freud cited Mantegazza's report in his work Über Coca (1884). That paper concerned the studies of the effects on man of cocaine, the alkaloid extracted from the coca leaves by the chemist Albert Niemann in 1859, rather than coca leaves, themselves.
See Guiliano Dall'Olio, "Paolo Mantegazza: memoria sulle proprietà terapeutiche della coca," Riv. Ital. Med. Lab., 7 (2011) 228-239.

Through many editions and translations of his three main works of sexuality (cited in this database) Mantagazza may have been the most widely read author on sexuality in the 19th century. He was also a widely published author on various other topics.

See V. Sigusch, "The birth of sexual medicine: Paolo Mantegazza as pioneer of sexual medicine in the 19th century," J. Sex. Med., 5, 217-222.

Digital facsimile of the 1875 second edition from Google Books at this link.

Digital facsimile of the Milan, 1891 edition from the Internet Archive at this link.

Mantegazza performed pioneering research into the physiology of pain at his experimental laboratory. His work marks the beginning of algometry, the scientific measurement of responses to pain stimuli.

Mantegazza invented an “algometer,” a device for measuring the physiological effects of pain in laboratory animals, and also investigated the facial expressions of individuals subjected to painful stimuli. His pain researches, conducted during the 1860s and 1870s, are summarized in his Fisiolgia del dolore.

Digital facsimile of the 1886 edition from the Hathi Trust at this link.

Translated into English "from the latest Italian edition, as approved by the author, by Samuel Putnam, edited with an introduction by Victor Robinson" as Sexual relations of mankind, New York: Eugenics Publishing Company (1935).