

Science
The science and chemistry of salvia has just recently been seriously tested by scientists and researchers. As for the hallucinogenic effects, the ingredient "Salvinorin A" is what does the trick. Salvinorin A is the main salvia content and the main salvia ingredient. The science of salvia continues to draw more attention as news stories look into the use and effects of salvia. As years go on, salvia enthusiasts will continue to study the salvia science and salvia chemistry, providing more information for labs and tests.
When looking at it scientifically, its a chemical called neoclerodane diterpene found primarily in the leaves that is psychoactive. Nothing has been found in the stems that has a chemically psychoactive effects. Through salvia science and salvia chemistry, your body needs a dose of 200-500 milligrams of pure salvia in order for the salvia effects to become psychoactive. The results usually take effect within 30 seconds and can last up to 30 minutes.
What scientists have looked deep into are the opioid receptors which are widely distributed in the brain, spinal cord, and in pain neurons. Selective kappa opioid receptor agonists have definite hallucinogenic effects which are generally undesirable in medical and pharmaceutical drugs. The reason being is that these receptors could have frightening or disturbing effects in tested humans. The neocerodane diterpene kappa opioid receptor agonist is found in the Salvinorin A. To set salvia apart from other hallucinogens, Salvinorin A does not activate the serotonin 2A receptor, which mediates the effects of other schedule I hallucinogens.
If you look into the chemistry and science of other sage the one main ingredient that isn't present is salvinorin. As said before salvinorin is the main compound that creates the hallucinogenic properties. Scientifically when we mention neocerodane diterpene it means that it only contains carbon, hydrogen, and oxygen atoms. This makes up 96% of salvinorin, which is the salvinorin A. The rest, 4% is salvinorin B which is not known to have any psychoactive effects. So in the eyes of salvia science and salvia chemistry, the more salvinorin A, the more hallucinogenic properties there are.
Looking into the salvia science and comparing to a drug is quite interesting. Salvia divinorum can't be classified as a drug due to a couple facts. For one, salvinorin A doesn't have any opiates when broken down. It also doesn't contain alkaloids. Alkaloids contain nitrogen atoms which come up positive on drug tests. Salvinorin is a diterpene which contains no nitrogen. All in all, if you were to take a urine test, salvia would not produce a positive reaction for both opiates or other alkaloid drugs.
So you're probably wondering how salvia works in our brain. Well through chemical and scientific salvia testing, salvinorin A is still unknown to why it has psychoactive properties, but there is a reason for its neurological reaction. As mentioned in the previous paragraph, salvinorin A is a kappa opioid receptor agonist. These receptors are known for binding and triggering activity in proteins throughout your brain and spinal cord.
Interesting enough, salvia doesn't activate "mu receptors" that most opiate drugs do. This is the reason why salvia is not addictive. Much like opiates, they have a strong efect on mu receptor but not on kappa receptors. This is the reason why opiates don't have strong visionary effects, but are very addicting. Salvia has extremely strong visionary effects, but is not addicting.
In a nutshell this is the science of salvia and the chemistry of salvia. Due to it's extreme popularity there is much more to come as far as researching and studying. You can also see why salvia has become very confrontational in people comparing it to a legal high. You can also see why the "salvia legal high" title can really be a difficult argument for both salvia pro/con parties..