Prostate cancer relies on testosterone to promote growth.
Blocking or removing the source of testosterone causes the tumour to shrink.
Hormonal control can be achieved with surgical or medical castration, which involves removing the testicles or controlling their production of male hormones through prescription drugs. 

Most men now choose a depot injection of an LHRH agonist. This is injected subcutaneously (under the skin) once every three months and is as effective as surgical castration. The Urologist usually performs the first injection, then after this they can be done by the patient’s General Practitioner.

Hormonal therapy can be used to target cancer that has spread beyond the prostate gland and is thus beyond the reach of local treatments such as surgery or radiation therapy. This treatment is also helpful in relieving the painful symptoms of advanced disease. In addition, it is being investigated as a way to prevent cancer growth before it has a chance to metastasize. Although hormonal therapy cannot cure prostate cancer, it will usually shrink or halt the advance of the disease — for several years. 

Hormonal treatment causes local prostate cancer and metastatic disease to regress (shrink) and PSA levels to fall. However, both methods can cause complications, including hot flashes, impotence and a loss of libido. Hormonal drug therapy can sometimes cause mild breast enlargement. Long-term hormone treatment also may cause weight gain and loss of bone density.