Stimulants are drugs that can reduce fatigue, suppress appetite, and increase alertness and aggressiveness. They stimulate the central nervous system, increasing your heart rate, blood pressure, body temperature and metabolism.

The most common stimulants include caffeine and amphetamines (Dexedrine, Benzedrine). Cold remedies often contain the stimulants ephedrine, pseudoephedrine hydrochloride (Sudafed) and phenylpropanolamine (Acutrim). Street drugs such as cocaine and methamphetamine also belong to this group.

Although stimulants can boost physical performance and promote aggressiveness on the field, they have side effects that can impair athletic performance. Nervousness and irritability make it hard to concentrate on the game, and insomnia can prevent an athlete from getting needed sleep. Athletes may become psychologically addicted or develop a tolerance so that they need greater amounts to achieve the desired effect.

Other side effects include:

Heart palpitations

Heart rhythm abnormalities

Weight loss

Mild hypertension

Hallucinations

Convulsions

Brain hemorrhage

Heart attack and other circulatory problems

Creatine monohydrate is a compound produced by your body that helps release energy in your muscles. Creatine is a naturally occurring compound — you can ingest creatine from protein-rich foods such as meat or fish, or you can take a nutritional supplement. Supplements are available over the counter.

Unlike androstenedione, scientific research indicates that creatine may have some benefit — it can produce small gains in short-term bursts of power.

"Most of the research points to small improvements in short-term power activities like improving maximum-weight bench press or increasing speed during cycling sprints of very short duration," says Edward Laskowski, M.D., a physical medicine and rehabilitation specialist at Mayo Clinic, Rochester, Minn., and co-director of Mayo Clinic's Sports Medicine Center. "Some studies have shown an increase in lean muscle mass with creatine. As a result, we've got a lot of press on creatine producing steroid-like results without the side effects."

Creatine helps muscles make and circulate more adenosine triphosphate (ATP). ATP is used for quick, explosive bursts of activity, as in weightlifting or sprinting. Creatine also reduces energy waste products — such as lactic acid — that can cause muscle fatigue. As a result, creatine is purported to enhance performance and decrease fatigue. But there's no evidence that creatine enhances performance in aerobic or endurance sports.

Your liver produces about 2 grams of creatine each day. You can also get creatine from the meat in your diet. Creatine is stored in your muscles, and levels are relatively easily maintained. Because your kidneys remove excess creatine, the value of supplements to someone who already has a high muscle creatine content is questionable.

Possible side effects of creatine that can decrease athletic performance include:

Stomach cramps

Muscle cramps

Nausea

Vomiting

Diarrhea

Weight gain is a known side effect of creatine — one that is sought after by athletes who need to increase their size. But with prolonged creatine use, weight gain is more likely the result of water retention than an increase in muscle tissue. Water is drawn into your muscle tissue, away from other parts of your body. This puts you at risk of dehydration.

High-dose creatine use may potentially damage your:

Kidneys

Liver

Heart

It's unknown what kind of effect taking creatine has over the long term, especially on teens or younger children. Dosage levels vary widely, depending on which product you use and how much creatine you take.

Since creatine isn't regulated by the Food and Drug Administration (FDA), you can't be sure of the purity of creatine supplements you buy on the market. Studies have found varying mixtures of creatine in different creatine products. And some of the inactive ingredients mixed in with the creatine may cause significant side effects, such as allergic reactions.

The bottom line is that the safety of taking creatine is questionable. Most studies involving creatine use examine the performance-enhancing aspects, and side effects are generally reported only anecdotally.

Androstenedione (andro) is a hormone produced by the adrenal glands, ovaries and testes. It's a precursor hormone that's normally converted to testosterone and estradiol in both men and women.

Manufacturers of synthetic androstenedione, through vigorous marketing efforts, have claimed that their products increase your body's production of testosterone. According to proponents of andro supplements, an elevated level of testosterone allows athletes to train harder and recover more quickly.

Scientific studies that refute these claims are now emerging. In fact, these studies show that supplemental androstenedione doesn't increase testosterone and that your muscles don't get stronger with andro use.

On Oct. 22, 2004, the Anabolic Steroid Control Act of 2004 classified andro as a controlled substance, adding it to the list of banned anabolic steroids and making its use as a performance-enhancing drug illegal.

Side effects of andro differ for men and women. In men it can actually decrease the production of testosterone while increasing the production of estrogen. Side effects in men include acne, diminished sperm production, shrinking of the testicles and enlargement of the breasts. In women, side effects include acne and masculinization, such as deepening of the voice and male-pattern baldness. Andro might also stunt your child's growth.

In men and women, supplemental androstenedione can decrease high-density lipoprotein (HDL) cholesterol (the "good" cholesterol). Lower HDL levels put you at greater risk of heart attack and stroke.