Stimulants create a fast rush of euphoria and energy, usually by speeding up cognitive processing and the central nervous system. Breathing and heart rate increase, thoughts race, minds speak. Although generally lacking in visual effects, many stimulants cause a general feeling of well being, and the increased thought processing allows for greater enjoyment of your surroundings, including music. Because of the increase in breathing an heart rate, most stimulants become quite dangerous at excessive doses. In moderation and at recreational doses however, most are relatively safe.
Most stimulants work by causing the brain to increase the level of neurotransmitters available for use (i.e. Seretonin and Dopamine). Once their effects wear down however, the user experiences a depressive crash because of the sudden reversal of effects. This often leads to redosing, which can quickly become habitual and very addicting. Stimulant users should have strong will power, and the ability to tell themselves 'no', and stick with it - Less they wish to join the many individuals that have succumbed to addiction.
A good number of stimulants are derived from plants. So naturally the plants themselves also have a stimulant effect. Though not generally as strong as any purified stimulant, the plants are very safe to use as a stimulant on a frequent basis and can generally be enjoyed free of major side effects.
Methylzanthines are a group of 3 mild stimulants based upon the Xanthine molecule. Best known amongst them is Caffeine, which is the only Xanthine to have recreational value. The remaining two are found naturally in chocolate and tea and have high toxicity if attempted to be used recreationally. All Xanthines are diuretic (that is, they increase frequency of urination).
Sympathomimetic drugs include the common stimulants that everyone has heard so much about in the media. From Cocaine, to Ecstacy, to Amphetamines, to Crystal Meth. Almost every recreational stimulant is a part of this subclass.
Amphetamines are not a true class of drug, but rather a way of a grouping several drugs with similar effects together. Amphetamines work primarily by releasing stored dopamine, norepinephrine and seretonin in the brain, as well as blocking the reuptake of dopamine and norepinephrine. This combines a feeling of pleasure (Seretonin) with a speedy rush of energy (Norepinephrine is very similar to adrenaline).
See Psychedelic Empathogens - These are the same drugs.