Because the older body usually has less water in it, alcohol does not become diluted as much as it used to so its effects are stronger. The higher the alcohol concentration, the faster the blood alcohol level will increase. The liver can normally metabolize one drink per hour with limited physical and mental effects on the user. Drugs that alter the chemistry of the brain and nervous system are called psychoactive agents. A psychoactive agent can act as a stimulant, a depressant, or a hallucinogen.
People use stimulants to stay alert, to get a rapid surge of energy, or to lose weight.
Caffeine and nicotine both stimulate the central nervous system. Depressants include sedatives, sleep aids and painkillers. Alcohol is the most widely used depressant, although many people think it's a stimulant since it decreases people's inhibitions. Hallucinogens are chemical agents that alter a person's sense of reality, enhancing and intensifying perception. PCP usually is combined with marijuana or other leafy substances such as oregano or parsley, rolled in a cigarette, and smoked.