The World Anti-Doping Agency (WADA) prepares The Prohibited List, the document that outlines which substances Olympic-bound athletes should not have in their systems. The first List (as it's known) was published in 1963, and it is updated every year.

These categories represent the latest updates to the List, as of January 1, 2008. Therefore, they are the guidelines for athletes in competing in Beijing. Some of these initiatives are still in their infancy, such as gene doping, but this is the latest word on what they are, what they do and why they are banned by the International Olympic Committee and WADA.

Prohibited Substances (in and out of competition):

  • Anabolic agents (natural or man-made)

Examples: testosterone, DHEA, nandrolone

What they do: These substances promote muscle and bone growth.

Why use them: They can improve the body's ability to train and compete at a high level by reducing fatigue and recovery time. Muscle and bone growth may lead to increased strength and power.

Side effects: Jaundice and liver damage, acne, heart problems, mood swings, depression, paranoia. Men may develop breast tissue and lose hair. Women may develop more facial and body hair, as well as irregular periods.

  • Hormones and related substances

Examples: erythropoietin (EPO), human growth hormone (hGH), insulins

What they do: These substances influence specific bodily functions.

Why use them: EPO stimulates production of red blood cells, which improves the amount of oxygen the blood can deliver to the muscles; hGH stimulates production of another growth factor, which in turn stimulates cartilage production for stronger bones and muscles; insulin can promote muscle growth and definition.

Side effects: EPO thickens the blood, which causes the heart to work harder and makes the blood more susceptible to clotting; hGH can lead to protruding jaw and eyebrow bones, as well as glucose intolerance; insulin can lead to low blood sugar levels, which can cause nausea, weakness, shortness of breath, coma and even death.

  • Beta-2 agonists

Examples: salbutamol, formoterol, terbutaline

What they do: Beta-2 agonists are bronchodilators, meaning they relax the muscles in the airways, which allows them to expand and use more air.

Why use them: They are commonly used to treat asthma symptoms such as wheezing, shortness of breath and coughing. However, if administered through the bloodstream, they can have similar effects to anabolic agents (see above).

Side effects: Headache, increased heart rate or palpitations, dry throat, nausea.

  • Hormone antagonists and modulators

Examples: aromatase inhibitors, selective estrogen receptor modulators (SERMs), myostatin inhibitors

What they do: Aromatase inhibitors and SERMs block estrogen production or inhibit its action in breast and other tissue; myostatin inhibitors promote muscle growth by blocking myostatin, which can impede muscle growth.

Why use them: Muscle growth, muscle growth, muscle growth.

Side effects: Bone loss.

  • Diuretics and other masking agents

Examples: diuretics, alpha-reductase inhibitors, plasma expanders

What they do: Diuretics combat water retention, which can cause weight gain, and dilute urine; alpha-reductase inhibitors and plasma expanders conceal the presence of banned substances in the blood or urine.

Why use them: Diuretics can help athletes maintain a certain weight to remain in a specific weight class; alpha-reductase inhibitors can conceal the presence of male hormones in the urine; plasma expanders can mask illegal substances in the blood.

Side effects: Diuretics: dehydration, dizziness, headaches, nausea, cramps and kidney and heart failure; alpha-reductase inhibitors: sexual dysfunction; plasma expanders: allergic reactions and anaphylactic shock.

Prohibited Methods (in and out of competition):

  • Enhancement of oxygen transfer (also known as blood doping or artificially enhancing uptake, transport or delivery of oxygen)

What it does: Adding blood or red blood cell products to the body to increase red blood cell mass.

Why do it: Increased red blood cell mass increases the amount of oxygen the blood can carry to muscles. This especially helps the stamina of endurance athletes.

Side effects: Elevated blood volume levels strain the heart, which has to work harder to pump blood. High volume levels also increase the risk of blood clots, heart attacks and stroke. Using someone else's blood can increase the risk of infection and the transmission of diseases such as HIV.

  • Chemical and physical manipulation (i.e. tampering with a urine sample, intravenous infusion)

Using someone else's urine to avoid having banned substances detected in your own urine test.

  • Gene doping

The use of cells, genes or genetic elements, or the modulation of gene expression to enhance athletic performance.

Prohibited substances (in competition):

  • Stimulants

Examples: most forms of ephedrine, amphetamine, cocaine

What they do: They jump-start the central nervous system to speed up the brain and body.

Why use them: Increase alertness, competitiveness and aggressiveness and reduce fatigue.

Side effects: Besides the risk of dependence, stimulants can cause a number of problems, such as increased heart rate, blood pressure and sweating, insomnia, heart palpitations, aggressive behaviour and reduced coordination and balance.

  • Narcotics

Examples: heroin, methadone, morphine

What they do: These are mostly painkillers, but they can also lead to feelings of euphoria or invincibility.

Why use them: These substances can boost an athlete's perception of his or her own athletic abilities in addition to masking the pain of an injury.

Side effects: Masking the pain of an injury while continuing to compete could result in greater injury. These substances are also addictive, and can cause a variety of symptoms from nausea and vomiting to respiratory complications and constipation.

  • Cannabinoids

Examples: marijuana, hashish

What they do: The most common effect of cannibinoids is relaxation and contentedness.

Why use them: Athletes may ingest cannabinoids recreationally, but few would take them for performance-enhancing purposes.

Side effects: Cannabinoids can lead to difficulty concentrating, impaired motor skills and reflexes, anxiety, mood swings and weight gain.

  • Glucocorticosteroids

Example: cortisone

What they do: These are powerful anti-inflammatory substances that treat arthritis, asthma, allergic reactions and inflamed joints.

Why use them: To treat injuries and reduce pain.

Side effects: These substances can cause fluid retention, hyperglycemia and mood disorders, as well as systemic infections and softening of connective tissue between bones and muscles.