Salvia divinorum: general

Salvia divinorum is a perennial mint-family plant native to the Sierra of Oaxaca, Mexico [1], also known as diviner's sage, magic mint, mystic sage, ska Maria, or ska Pastora [1]. The Mazatecs are reported to have used it for rheumatism, headaches, diarrhoea, and abdominal swelling, and for shamanic and divinatory purposes [1]. It was first recognised as a distinct species in 1962 [1].

The psychoactive effect comes from salvinorin A, a neoclerodane diterpene [2], with other related diterpenes also present in the plant [3]. Salvinorin A is not an alkaloid — unlike most classical hallucinogens, it contains no nitrogen atom [2]. It works differently too: it's a potent, selective full agonist at the kappa-opioid receptor, and this is thought to be primarily responsible for the hallucinogenic effect [2][4].

The most important thing to know: the toxicity of Salvia divinorum is currently poorly understood [2]. There is no solid human data on dosing, duration, or onset. What exists comes from animal studies, which reported rapid onset and short distribution and elimination half-lives [3], and no evidence of short- or long-term toxicity [3] — but animal pharmacokinetics and animal toxicity findings do not tell you what happens in a human body, and "no evidence of toxicity" in animal studies is not the same as "safe."

Given that gap, treat this as an unknown: start with far less than seems reasonable, never use it alone, and never combine it with anything else. There's also early interest in salvinorin A for chronic pain, addiction, and PTSD [4], but that's research-stage, not something that tells you anything about using the plant now.

References

1. Salvinorin A and Salvia divinorum: Toxicology, Pharmacological Profile, and Therapeutic Potential — PMC / MDPI
2. Salvia divinorum drug profile — European Union Drugs Agency (EUDA)
3. Salvia divinorum and salvinorin A: an update on pharmacology and analytical methodology — UC Berkeley Center for the Science of Psychedelics
4. A chromosome level reference genome of Diviner’s sage (Salvia divinorum) provides insight into salvinorin A biosynthesis — BMC Plant Biology / Springer Nature