While many prescription drugs can be abused or misused, these three classes are most commonly abused:
Opioids - often prescribed to treat pain.
CNS Depressants - used to treat anxiety and sleep disorders.
Stimulants - prescribed to treat narcolepsy and attention deficit/hyperactivity disorder.

Opioids are commonly prescribed because of their effective analgesic or pain relieving properties. Many studies have shown that properly managed medical use of opioid analgesic drugs is safe and rarely causes clinical addiction, which is defined as compulsive, often uncontrollable use. Taken exactly as prescribed, opioids can be used to manage pain effectively. Among the drugs that fall within this class - sometimes referred to as narcotics - are morphine, codeine, and related drugs.

Morphine is often used before or after surgery to alleviate severe pain. Codeine is used for milder pain. Other examples of opioids that can be prescribed to alleviate pain include oxycodone (OxyContin-an oral, controlled release form of the drug); propoxyphene (Darvon); hydrocodone (Vicodin); hydromorphone (Dilaudid); and meperidine (Demerol), which is used less often because of its side effects. In addition to their effective pain relieving properties, some of these drugs can be used to relieve severe diarrhea (Lomotil, for example, which is diphenoxylate) or severe coughs (codeine).

Opioids act by attaching to specific proteins called opioid receptors, which are found in the brain, spinal cord, and gastrointestinal tract. When these drugs attach to certain opioid receptors in the brain and spinal cord they can effectively block the transmission of pain messages to the brain. In addition to relieving pain, opioid drugs can affect regions of the brain that mediate what we perceive as pleasure, resulting in the initial euphoria that many opioids produce. They can also produce drowsiness, cause constipation, and, depending upon the amount of drug taken, depress breathing. Taking a large single dose could cause severe respiratory depression or be fatal.

Opioids may interact with other drugs and are only safe to use with other drugs under a physician's supervision. Typically, they should not be used with substances such as alcohol, antihistamines, barbiturates, or benzodiazepines. These drugs slow down breathing, and their combined effects could risk life-threatening respiratory depression. Chronic use of opioids can result in tolerance to the drugs so that higher doses must be taken to obtain the same initial effects. Long-term use also can lead to physical dependence - the body adapts to the presence of the drug and withdrawal symptoms occur if use is reduced abruptly.

Symptoms of withdrawal can include restlessness, muscle and bone pain, insomnia, diarrhea, vomiting, cold flashes with goose bumps ("cold turkey"), and involuntary leg movements. Methadone, a synthetic opioid that blocks the effects of heroin and other opioids, eliminates withdrawal symptoms, and relieves drug craving. It has been used for over 30 years to successfully treat people addicted to opioids. Other medications include LAAM (levo-alpha-acetyl-methadol), an alternative to methadone that blocks the effects of opioids for up to 72 hours. Naltrexone is a long acting opioid blocker often used with highly motivated individuals in treatment programs promoting complete abstinence, and also to prevent relapse.

CNS depressants slow down normal brain function. In higher doses, some CNS depressants can become general anesthetics. CNS depressants can be divided into two groups, based on their chemistry and pharmacology:
Barbiturates, such as mephobarbital (Mebaral) and pentobarbital sodium (Nembutal), which are used to treat anxiety, tension, and sleep disorders.

Benzodiazepines, such as diazepam (Valium), chlordiazepoxide HCl (Librium), and alprazolam (Xanax), which can be prescribed to treat anxiety, acute stress reactions, and panic attacks. Benzodiazepines that have a more sedating effect, such as triazolam (Halcion) and estazolam (ProSom) can be prescriped for short-term treatment of sleep disorders.
There are many CNS depressants, and most act on the brain similarly - they affect the neurotransmitter gamma-aminobutyric acid (GABA). Neurotransmitters are brain chemicals that facilitate communication between brain cells. GABA works by decreasing brain activity. Although different classes of CNS depressants work in unique ways, ultimately it is their ability to increase GABA activity that produces a drowsy or calming effect.

Despite these beneficial effects for people suffering from anxiety or sleeping disorders, barbiturates and benzodiazepines can be addictive and should be used only as prescribed. CNS depressants should not be combined with any medication or substance that causes sleepiness, including prescription pain medicines, certain over-the-counter cold and allergy medications, or alcohol.

The effects of the drugs can combine to slow breathing, or slow both the heart and respiration, which can be fatal. Discontinuing prolonged use of high doses of CNS depressants can lead to withdrawal. Because they work by slowing the brain's activity, a potential consequence of abuse is that when one stops taking a CNS depressant the brain's activity can rebound to the point that seizures can occur. Someone thinking about ending their use of a CNS depressant, or who has stopped and is suffering withdrawal, should speak with a physician and seek medical treatment.

Stimulants are a class of drugs that enhance brain activity - they cause an increase in alertness, attention, and energy that is accompanied by increases in blood pressure, heart rate, and respiration. Historically, stimulants were used to treat asthma and other respiratory problems, obesity, neurological disorders, and a variety of other ailments. As their potential for abuse and addiction became apparent, the use of stimulants began to wane. Now, stimulants are prescribed for treating only a few health conditions, including narcolepsy, attention-deficit hyperactivity disorder (ADHD), and depression that has not responded to other treatments.

Stimulants may also be used for short-term treatment of obesity, and for patients with asthma. Stimulants such as dextroamphetamine (Dexedrine) and methylphenidate (Ritalin) have chemical structures that are similar to key brain neurotransmitters called monoamines, which include norepinephrine and dopamine. Stimulants increase the levels of these chemicals in the brain and body. This, in turn, increases blood pressure and heart rate, constricts blood vessels, increases blood glucose, and opens up the pathways of the respiratory system. In addition, the increase in dopamine is associated with a sense of euphoria that can accompany the use of these drugs.

Research indicates that people with ADHD do not become addicted to stimulant medications, such as Ritalin, when taken in the form prescribed and at treatment dosages. However, when misused, stimulants can be addictive. The consequences of stimulant abuse can be extremely dangerous. Taking high doses of a stimulant can result in an irregular heartbeat, dangerously high body temperatures, and/or the potential for cardiovascular failure or lethal seizures. Taking high doses of some stimulants repeatedly over a short period of time can lead to hostility or feelings of paranoia in some individuals.

Stimulants should not be mixed with antidepressants or over-the-counter cold medicines containing decongestants. Anti-depressants may enhance the effects of a stimulant, and stimulants in combination with decongestants may cause blood pressure to become dangerously high or lead to irregular heart rhythms. Treatment of addiction to prescription stimulants, such as methylphenidate and amphetamines, is based on behavioral therapies proven effective for treating cocaine or methamphetamine addiction. At this time, there are no proven medications for the treatment of stimulant addiction. Antidepressants, however, may be used to manage the symptoms of depression that can accompany early abstinence from stimulants.

Steroids (Anabolic-Androgenic) The major side effects from abusing anabolic steroids can include liver tumors and cancer, jaundice (yellowish pigmentation of skin, tissues, and body fluids), fluid retention, high blood pressure, increases in LDL (bad cholesterol), and decreases in HDL (good cholesterol). Other side effects include kidney tumors, severe acne, and trembling.
In addition, there are some gender-specific side effects:

* For men--shrinking of the testicles, reduced sperm count, infertility, baldness, development of breasts, increased risk for prostate cancer.
* For women--growth of facial hair, male-pattern baldness, changes in or cessation of the menstrual cycle, enlargement of the clitoris, deepened voice.
* For adolescents--growth halted prematurely through premature skeletal maturation and accelerated puberty changes. This means that adolescents risk remaining short the remainder of their lives if they take anabolic steroids before the typical adolescent growth spurt. In addition, people who inject anabolic steroids run the added risk of contracting or transmitting HIV/AIDS or hepatitis, which causes serious damage to the liver.