Hormone replacement therapy (HRT) patches prevent the growth of prostate tumours by drastically lowering levels of testosterone.
Prostate cancer that is starting to spread is known to be fuelled by the male hormone.
Current treatments include injected drugs called LHRH agonists, which interfere with testosterone production but can have serious long-term side effects.
Oestrogen, the female sex hormone, taken in a pill form also blocks testosterone production, but can lead to dangerous blood clots and strokes.
However, the patches also lower testosterone but do not cause the same blood clotting problems.