Neurotransmitters are chemical substances that transmit nerve impulses across a synapse to another nerve, muscle, or gland. Psychoactive drugs can enhance or impede the activity of neurotransmitters. Most psychoactive drugs either mimic natural neurotransmitters or change the nerve receptors.
Plant-derived psychoactive drugs like cocaine, cause a stimulant effect by blocking the transport of dopamine away from the receptors and suppressing dopamine production. Alkaloids are extracted from plants and include peyote and herbal ecstasy which act as dopamine agonists and stimulants. Some alkaloids like LSD and psilocybin are serotonin agonists and antagonists which cause hallucinations. Alkaloids such as caffeine are stimulants. Nicotine raises dopamine while lowering MAO.
Synthetic psychactive drugs include tranquilizers and sedatives which inhibit serotonin reuptake and block catecholamine receptors. Stimulants like amphetamines and methamphetamine interfere with the reuptake and release of the natural neurotransmitters, dopamine and adrenaline, leading to possible hypertension or heart attack.
As you can see, psychoactive drugs can be harmful or helpful to the individual because of the neurological changes they cause.