What is human growth hormone?
Growth hormone is a powerful anabolic hormone that occurs naturally in the human body. It is produced by the pituitary gland in the brain and stimulates the growth of muscle, cartilage, and bone.
It is made throughout a person's lifetime but is more plentiful during youth. It stimulates growth in children and plays an important role in adult metabolism.
Scientists first isolated HGH in 1956. Three years later, NHS doctors began to use it in the treatment of children suffering from stunted growth.
Before the advent of genetic engineering, the only source of HGH was human corpses. The pituitary glands were removed from cadavers, processed and the hormones made available in injectable form.
Synthetic HGH can now be made in unlimited quantities in the laboratory. Its use in sport was banned in 1989 by the International Olympic Committee's medical commission.
Why might a sportsman take it?
In simple terms, to increase muscle size. Because there is a correlation between muscle size and strength, competitors in events that require power and short bursts of explosive strength would be most likely to benefit.
It also allows tired muscles to recover quicker - allowing you to train harder and more often.
But it is of far less use in endurance sports, where EPO has been favoured because it increases one's oxygen-carrying capacity and thus stamina.
Is HGH guaranteed to make you better at sport?
Not at all. There are many factors that make up sporting performance - physical strength is only one. And HGH has different effects on different people.
What are the potential side-effects?
Excess HGH in the body can cause acromegaly, a disease where the hands become spade-like in appearance as they get bigger. Growth of the facial bones causes the face to change shape too.
The jaw becomes larger, with spaces appearing between the teeth because of this, and the eyebrows become more prominent. The tongue enlarges and the skin becomes coarse and oily.
Organs like the heart, liver and kidneys will also undergo excessive growth, leading to potentially life-threatening problems - one of which is cardiomyopathy, a disease of the heart muscle.
The heart loses its ability to pump blood and, in some instances, heart rhythm is disturbed, leading to irregular heartbeats
There is also an increased risk of cancers due to the abnormal growth of cells.
What about CJD?
Creutzfeld-Jakob Disease is a side effect of the HGH that was made from the pituitary glands of dead people.
The infective agent which causes CJD resides in the brain, and using HGH in live patients ran the risk of passing on the agent. CJD has an incubation period of up to 30 years.
As a result, those who have received treatment may not know their fate for a long time. Synthetic HGH is clear of the infective agent that causes CJD.
Is HGH always used alone?
No. There is anecdotal evidence to suggest that it is sometimes used as part of a cocktail of banned drugs, sometimes with steroids.
Why is it so hard to test for?
HGH occurs naturally in the body, which makes it hard to distinguish between what is produced regardless of outside interference and what is an administered dose.
And it is almost impossible to set a blood level of HGH that would be considered unnaturally high and indicative of doping, because levels of naturally-occurring HGH can vary by more than 100-fold in response to factors such as nutritional state, sleep and exercise.