Stimulants are a class of drugs that increase alertness and physical activity. They include amphetamines (pharmaceuticals such as Dexedrine and Biphetamine also known as speed), methamphetamines (meth, chalk, ice, crystal, glass) and cocaine (blow, bump, coke, Charlie, rock, snow, toot). If you start using these drugs, chances are high that you will develop intense cravings for more.
Stimulants may be taken in different forms: swallowed, snorted, smoked, or injected. Injecting stimulants is the riskiest method (due to HIV needle infection, and so on) and swallowing is the safest- from immediately physical viewpoint, at any rate.
Stimulants increase heart and respiration rates, increase blood pressure, dilate (expand) the pupils, and decrease appetite. The last side effect was once a sought-after attribute of stimulants in their use as weight-loss drugs. Now we know better!