Many drugs can produce more than one effect, depending on the quantity taken. Behavior stimulants - cocaine caffeine, nicotine. Narcotic analgesics - opium, morphine, codeine 4. Psychedelics - THC, mescaline, atropine, scopolamine, LSD 5.
Most psychoactive compounds contain nitrogen and most are alkaloids.
THC, alcohol, LSD and hallucinogenic terpenes are exceptions. Absorption - orally, injection, membrane absorption. Most act on CNS, drugs reach brain faster than other tissues but passage of chemicals into brain tissue is more difficult than in other parts of body. Alter interactions between neurons. Neurons transmist information chemically via neurotransmitters which can cross the synapse between neurons.