brain on drugs

Here are summaries of the effect of select street drugs on the brain. Some of the introductory information is derived from About.com. Select authoritative references for information about effects of drugs on the brain include:
Drug Abuse in the Decade of the Brain, Gabriel G. Nahas and T. F. Burks, Eds., IOS Press, 1997.
Drug Use and Abuse, Jean-Pierre Changeux, in The Brain by G. M. Edelman and J. Changeux, editors, Transaction Publishers, 2001.

Heroin

Heroin is a highly addictive opiate (like morphine). Brain cells can become dependent (highly addictive) on this drug to the extent that users need it in order to function in their daily routine. While heroin use starts out with a rush of pleasure, it leaves the use in a fog for many hours afterwards. Users soon find that their sole purpose in life is to have more of the drug that their body has become dependant on.
From Dakota State University, a great resource on the effects of various drugs on the body and brain.
See also, National Institute on Drug Abuse facts about heroin.

Marijuana

The parts of the brain that control emotions, memory, and judgment are affected by marijuana.  Smoking it can not only weaken short-term memory, but can block information from making it into long term memory. It has also been shown to weaken problem solving ability.
Cannabis and Cognitive Functioning, Nadia Solowi, Cambridge Univ. Press, 1998.

Alcohol

Alcohol is no safer than drugs. Alcohol impairs judgment and leads to memory lapses. It can lead to blackouts. It distorts vision, shortens coordination, and in addition to the brain can damage every other organ in the body.

Cocaine

Cocaine, both in powder form and as crack, is an extremely addictive stimulant. An addict usually loses interest in many areas of life, including school, sports, family, and friends. Use of cocaine can lead to feelings of paranoia and anxiety. Although often used to enhance sex drive, physical effect of cocaine on the receptors in the brain reduce the ability to feel pleasure (which in turn causes the dependency on the drug).
Cocaine: Effects on the Developing Brain, (Annals of the New York Academy of Sciences), John A. Harvey and Barry E. Kosofsky, Eds., New York Academy of Sciences, 1998.
The Neurobiology of Cocaine Addiction: From Bench to Bedside, Herman Joseph and Barry Stimmel, Eds., Haworth Press, 1997.
See National Institute on Drug Abuse facts about cocaine and crack.

Inhalants

Inhalants, such as glue, gasoline, hair spray, and paint thinner, are sniffed. The effect on the brain is almost immediate. And while some vapors leave the body quickly, others will remain for a long time. The fatty tissues protecting the nerve cells in the brain are destroyed by inhalant vapors. This slows down or even stops neural transmissions. Effects of inhalants include diminished ability to learn, remember, and solve problems. 
See National Institute on Drug Abuse facts about inhalants.

Ecstasy

Extended use of this amphetamine causes difficulty differentiating reality and fantasy, and causes problems concentrating. Studies have found that ecstasy destroys certain cells in the brain.  While the cells may re-connect after discontinued use of the drug, they don't re-connect normally. Like most drugs, this one impairs memory and can cause paranoia, anxiety, and confusion.
See National Institute on Drug Abuse facts about ecstasy.

LSD

While some people use LSD for the sense of enhanced and vivid sensory experience, it can cause paranoia, confusion, anxiety, and panic attacks. Like Ecstasy, the user often blurs reality and fantasy, and has a distorted view of time and distance.
See National Institute on Drug Abuse facts about LSD.

Steroids

Anabolic steroids are used to improve athletic performance and gain muscle bulk. Unfortunately, steroids cause moodiness and can permanently impair learning and memory abilities.
See National Institute on Drug Abuse facts about steroids.

Tobacco

Tobacco is a dangerous drug, putting nicotine into your body. Nicotine affects the brain quickly, like other inhalants, producing feelings of pleasure, like cocaine, and is highly addictive, like heroin.
See National Institute on Drug Abuse facts about nicotine.

Methamphetamine

Known on the street as meth, speed, chalk, ice, crystal, and glass, methamphetamine is an addictive stimulant that strongly activates certain systems in the brain. 
See National Institute on Drug Abuse facts about methamphetamine.

Ritalin

This drug is often prescribed to treat attention deficit disorder. It is becoming an illicit street drug as well. Drug users looking for a high will crush Ritalin into a powder and snort it like cocaine, or inject it like heroin. It then has a much more powerful effect on the body. It causes severe headaches, anxiety, paranoia, and delusions.