According to the International Society of Sports Nutrition, creatine (monohydrate) is the most effective ergogenic (performance-enhancing) nutritional supplement currently available to athletes for increasing high-intensity exercise capacity and lean body mass during training. A variety of forms are available and are discussed in the complete summary, but have not been shown to exert significant benefits over basic monohydrate supplementation.
Creatine's main action in the body is to store high-energy phosphate groups in the form of phosphocreatine. During periods of stress, phosphocreatine releases this energy to aid cellular function. This mechanism of action is what causes creatine to increase strength, but can benefit almost every body system, including the brain, bones, muscles, and liver. Most benefits of creatine occur through this energy mechanism.
Creatine is produced naturally in the body, and it is also found in foods (mostly meats, eggs, and fish; some in dairy).
Creatine has been shown to increase DHT (dihydrotestosterone) levels by 40% with a dosage of 5g per day. DHT is directly involved in hair loss in men, so long-term creatine usage could accelerate hair loss.
Creatine supplementation at normal dosages and with adequate hydration has been shown to have no harmful effects in any population tested (see its safety profile). The only observed side effects are stomach cramping if consumed with insufficient water, and diarrhea if too much is consumed at once. Controlled usage of creatine with adequate water may actually reduce cramping over the long term.