How does chemical castration work? Step-by-step guide to the process - as the UK government plans to roll the treatment out to paedophiles and sex criminals in Britain

Daily Mail​

22-05-2025

The UK government has revealed plans to chemically castrate paedophiles and sex criminals in Britain.
The process aims to drastically reduce an offender's sex drive and interest in sexual behaviour.
And previous studies have shown that it significantly reduces the risk of sex criminals re-offending.
The exact chemicals used during the process vary around the world, but trials in UK prisons currently use three types of medication to produce this effect.
The first two chemicals are anti-androgens that target the body's supply of testosterone - the male sex hormone which has a strong impact on the sex drive.
'They lower testosterone levels and result in reduced physiological signs of sexual arousal - fewer erections, less frequent ejaculation,' explained Belinda Winder, Professor of Forensic Psychology at Nottingham Trent University.
'Plus we see a reduction in people's levels of sexual fantasies.'
The UK also treats offenders with antidepressant medication called SSRIs which can permanently disrupt the libido and reduce sexual desire.
How does chemical castration work?
Chemical castration aims to reduce the risk of sex criminals re-offending by limiting or totally removing their sex drive and sexual desire.
This is mainly achieved by interfering with the body's natural production of sex hormones, or androgens.
In men, the two main androgens are testosterone and dihydrotestosterone (DHT) which are mainly produced in the testicles.
Several studies have shown that violent sexual offenders have higher levels of androgens and that high androgen levels correlate with both violence and sexual aggression.
So, by limiting the body's production and uptake of androgens, chemical castration can also reduce the offender's tendency towards violence and sexual activities.
In the UK, prisoners who have voluntarily chosen chemical castration are given two different chemicals collectively called 'anti-androgens'.
Professor Winder explained: 'Anti-androgens reduce the production of testosterone and have a more direct effect.
The first of these chemicals are 'androgen inhibitors' which bind to the testosterone receptors in the body, mainly in the testicles, and prevent testosterone binding and having its normal effect.
The second type of drug, called 'Gonadotropin-Releasing Hormone Agonists' (GnRH Agonists), works alongside the androgen inhibitors to bring down the level of testosterone in the body.
In a healthy person, the pituitary gland in the brain produces hormones which tell the testes to make more testosterone when levels drop below a certain level.
The GnRH agonists prevent the normal feedback loop, so the pituitary gland doesn't tell the testes to make any more androgens.
After treatment with these chemicals, testosterone levels typically drop to pre-pubescent levels after about five days.
In addition to these two drugs, the UK also uses antidepressant drugs called selective serotonin reuptake inhibitors (SSRIs).
These drugs are typically used to treat depression, and the chemical castration they cause is essentially a side-effect of this.
Professor Winder says: 'SSRIs increase levels of serotonin, thereby inhibiting physiological and psychological arousal, erection and orgasm.
'SSRIs also affect people psychologically - they provide people with more control over their sexual urges, compulsive behaviours and compulsive thinking.
'They help people break the cycle of sexual addiction.'
Is chemical castration permanent?
Chemical castration is not strictly permanent but some of the side-effects can be long lasting.
The treatment must be given at regular intervals via an injection or implants under the skin to keep testosterone levels low and serotonin levels high.
In similar programmes in South Korea, this costs around £2,700 annually for medication and monitoring, but costs are likely to vary nationally.
The effects of anti-androgen treatment will normally reverse if the offender stops taking the medication.
However, the side effects of long-term use include infertility, shrinking testicles, hot flushes, increased risk of cardiovascular disease, and osteoporosis.
Since chemical castration patients typically receive treatment for a minimum of three to five years, the risk of these effects is increased with time.
SSRI treatment is also generally reversible, with normal sexual function returning after stopping treatment.
However, SSRIs can cause persistent sexual dysfunction lasting for several years after stopping treatment.
Is chemical castration effective?
Chemical castration is supposed to be a treatment, not a punishment.
The goal is to help offenders overcome psychological conditions such as problematic sexual arousal rather than harm them by taking away their sex drive.
And, when delivered alongside psychological treatments, there is good evidence that these chemicals can be an effective form of treatment.
'They help people regain control over their thinking and sexual behaviour and what may have become an addiction to sexual "highs",' says Professor Winder.
Together, the medication in chemical castration helps offenders to 'change the channel away from unhealthy and illegal sexual interests'.
There is already a growing body of evidence that treatment with anti-androgens or SSRIs can significantly reduce re-offending rates.
Some studies suggest that the rate of re-offending in people treated with anti-androgens is as low as two to five per cent, compared with expected rates of 50 per cent.
Professor Winder's own research has shown that both anti-androgens and SSRIs can be clinically effective in treating problematic sexual arousal in over 30 per cent of cases.
The evidence for the effectiveness of SSRIs as a form of chemical castration is still a little less certain.
In the UK, there still isn't enough robust evidence that SSRIs work for chemical castration so physicians in the prison system have to prescribe them 'off label'.
Professor Winder adds: 'The evidence base for SSRIs is something myself and others have been evaluating for some years now, there is a growing evidence base that SSRIs are effective at doing so but we are in the process of collecting more evidence currently.'
Professor Winder adds that she would 'definitely' support Justice Secretary Shabana Mahmood's plan to expand the voluntary pilot scheme.
The UK government is reportedly considering joining Kazakhstan as one of the few countries where chemical castration could be mandatory. Pictured: Convicted paedophiles in Kazakhstan facing mandatory chemical castration
However, it is also worth noting that the current evidence supporting chemical castration is based on offenders who voluntarily opted for the treatment.
These patients usually received psychological therapy, which they actively and willingly took part in.
Research suggests that maximum effectiveness from chemical castration may only be achieved in sex offenders who are willing to work toward changing their perceptions and behaviours.
Ms Mahmood is also considering whether to make the measure mandatory, rather than voluntary.
This would make the UK an outlier among the countries where chemical castration is practised, and there is much less evidence to support the benefits of mandatory castration.
What is testosterone?
Testosterone is the male sex hormone and is mostly made in the testicles, but also in adrenal glands, which are near the kidneys.
It causes the voice to deepen, body hair to grow and the genitals to become larger during puberty.
As well as affecting sex drive and sperm production, it also plays a role in developing strong bones and muscles, and how the body distributes fat.
Women also create small amounts of the hormone in the ovaries and adrenal glands, and it affects their fertility and bones and muscles.
Testosterone levels which are too high or too low can cause various problems.
Low testosterone in men can cause erection problems, low sex drive, infertility, weakened muscles and bones, body fat gain and hair loss.
Too much testosterone, however, can trigger puberty in boys under the age of nine, is linked to aggression, and can increase the risk of prostate problems, including cancer.
Male testosterone levels tend to be highest when he is around 20 years old, and decline naturally with age.