What is chemical castration? Here's why doctors will be uneasy about using it on sex offenders

Chemical castration has been used on sex offenders in several countries over many years, with some success - but it remains controversial.

The principle of treatment, when used on men, is to inhibit the action of the sex hormone testosterone. The expectation is that the offender will have fewer sexual fantasies and a lower sex drive.

Many of the drugs are more widely used to treat prostate cancer, where they reduce the growth of hormone-sensitive tumours. But they vary in how effective they are at treating sex offenders - and how swiftly they act.

Sky News was given exclusive access to a clinical trial in Stockholm, Sweden, of a drug called degarelix. Results showed that just two weeks after the first injection, men - who were living in the community - had a significantly reduced risk of committing child sexual abuse.

But like other clinical trials, it was relatively small with only 52 men taking part. The evidence base for chemical castration is limited.

The drugs can also have side effects, including the development of breast tissue in men, depression and hot flushes. And some criminologists question whether lower reoffending rates really are as a result of treatment.

It's possible that men offered a shorter prison sentence in exchange for taking a drug to suppress testosterone may simply be motivated to stay out of prison when they are released. That hasn't stopped many countries using chemical castration.

Germany, Denmark and the UK have used it on men who volunteer for treatment. Read more:Chemical castration for sex offenders to be consideredMen's prisons 'could run out of space in months' In England, a trial has been running in prisons in the South West since 2022 - and is now being cited by the Home Office as a reason to expand a treatment programme to 20 regions around the country.

But the drugs have been used in the UK far longer. In 2012, it was revealed that around 100 sex offenders were being treated at HMP Whatton in Nottinghamshire, either with chemical castration or with drugs such as Prozac, which can alter patterns of thinking.

Justice Secretary Shabana Mahmood is reported to be considering making treatment compulsory for serious sex offenders. That would be deeply controversial with the medical profession in the UK.

Consent is a long-standing principle of treatment with any procedure. But in the US, chemical castration is mandatory for some men in several states.

California was the first, amending its penal code in 1996 to include it as a punishment for those convicted of sexual abuse of minors. Florida, Texas, Louisiana are among those that followed suit.

Poland, Russia, Moldova, and Estonia have also introduced compulsory treatment for paedophiles. But in the UK, without strong evidence on the risks and benefits of using the drugs, there will be professional unease about forcing men to undergo castration - even if they are convicted of the most heinous sexual offences against children..