Self-harm is a serious issue for women in prison, who make up only 5% of the prison population but account for half of all self-harm incidents, say Oxford University researchers.
According to a study in The Lancet, nearly a quarter of female prisoners cut, scratch or poison themselves.
Self-harm was also found to be a strong risk factor for suicide in prison, particularly among men.
Experts say more should be done to reduce self-harm rates in prisons.
The Oxford research team looked at self-harm incidents in all prisons in England and Wales between 2004 and 2009.
It found that 5% to 6% of male prisoners and 20% to 24% of female prisoners deliberately harmed themselves every year, resulting in 20,000 to 25,000 incidents per year.
This compared with a 0.6% rate among the UK’s general population.
Repeated self-harming was common, the study said, and a small group of 102 women inmates self-harmed more than 100 times a year.
Cutting and scratching were the most frequent methods of self-harming in men and women, followed by poisoning and overdosing.
The study also examined those at greatest risk of self-harm. In female prisoners, being younger than 20 years old, white, in a mixed local prison, or serving a life sentence were major factors.
In male prisoners, those at risk tended to be young, white, in a high-security prison and with a life sentence or unsentenced.
Prisoners who self-harmed were found to be at “substantially greater” risk of suicide than other inmates, particularly among men.
Older male prisoners (aged 30 to 49) with a history of serious self-harm were most at risk.
Dr Seena Fazel, joint study author from the department of psychiatry at the University of Oxford, said all prisoners who are self-harming should be regarded as a risk.
“Now we know the extent to which the risk of subsequent suicide in prisoners who self-harm is greater than the general population, suicide prevention initiatives should be changed to include a focus on prisoners who are self-harming, especially repeatedly.”