Some hormones occurring naturally in the human body, in particular the male sex hormone testosterone, have been modified so that their main property is to increase muscle bulk. These modified steroids are known as anabolic steroids, ‘Anabolic’ meaning ‘to chemically build up’.

Only two anabolic steroids are available on prescription in the UK, namely nandrolone and stanozol. These are marketed as Durabolin and DecaDurabolin and Stromba respectively. Some illicit anabolic steroids are, however, imported from abroad, and are therefore likely to have other names.

Anabolic steroids increase the retention of nitrogen by the body, allowing this to be used to build muscle. An athlete's strength will not be increased unless this is also combined with intensive training and a special high protein high calorie diet. Steroids also aid the production of red blood cells, which carry oxygen around the body. This has led to the belief that they might be helpful in increasing endurance.

There is however no evidence to support this.

Traces of anabolic steroids can be detected in the urine for an average of 3-6 months after taking the drug. This period can be considerably longer.

The intention behind the use of these steroids is that, by building up muscle bulk, they will also increase strength. They do have a limited medical role, such as treating patients for weight loss associated with severe continuous illness. Anabolic steroids are also used illicitly by athletes who take them to improve performance.