Kratom (Mitragyna speciosa) is the only opium substitute that really is a substitute. The pharmacological effects are mainly due to two alkaloids, Mitragynine and 7-hydroxymitragynine. Both are active at the mu-opioid receptor, and thus have morphine like effects. Below are links to papers for further reading. Will be updated as I read more.
What is Kratom? [Link]
DEA "Fact" Sheet [Link]
Chemistry and Pharmacology of Analgesic Indole Alkaloids from the Rubiaceous Plant, Mitragyna speciosa. Chem. Pharm. Bull. 52(8) 916—928 (2004) Hiromitsu Takayama [Link]
Comparison to Morphine
Of the two alkaloids with opioid effects, mitragynine and 7-hydroxymitragynine, the latter is more potent than morphine.
Pharmacological Studies on 7-Hydroxymitragynine, Isolated from the Thai Herbal Medicine Mitragyna speciosa: Discovery of an Orally Active Opioid Analgesic. Kenjiro Matsumoto 2006 [Link]
Antinociception, tolerance and withdrawal symptoms induced by 7-hydroxymitragynine, an alkaloid from the Thai medicinal herb Mitragyna speciosa. Kenjiro Matsumoto, Syunji Horie, Hiromitsu Takayama, Hayato Ishikawa, Norio Aimi, Dhavadee Ponglux, Toshihiko Murayama, Kazuo Watanabe [Link]
(Note if someone can find a free pdf of this document leave me the URL in the comments)
ETHNOPHARMACOLOGY OF KRATOM AND THE MITRAGYNA ALKALOIDS. KARL L.R. JANSEN and COLIN J. PRAST. Journal of Ethnopharmacology, 23 (1988) 115-119 [Link]