The Mirror reports today: "100 paedophiles are chemically castrated in jail." The story claims that:

• A treatment programme involving 100 prisoners at Whatton prison, a specialist prison for sex offenders in Nottinghamshire, has seen them undergo chemical "castration".

• The drug used is called leuprorelin, which is marketed as Prostap. It reduces the prisoners' testosterone levels to those of prepubescent children.

• The scheme is entirely voluntary.

• The scheme has government backing. The Ministry of Justice said: "We support the use of pharmaceutical interventions for high risk sex offenders given the evidence that it can be useful in reducing risk for some perpetrators."

What is the evidence that such programmes work? Where has the treatment been used before? Does hormone suppression really amount to "chemical castration"? And are sex offences really about libido, or violence?

I'm going to look at the evidence and talk to the experts. To be totally clear the information I have so far is from the Mirror's somewhat sensationalist coverage of this. I will be talking with Don Grubin, the professor of forensic psychiatry who has been running the programme, this morning to get the full facts of the programme as well.



Can you suggest any evidence I should consider? Do get in touch below the line, via Twitter at @pollycurtis or email me at polly.curtis@guardian.co.uk.

Analysis

The Mirror story, as well as this Guardian Q&A on chemical castration, both mention Scandinavia studies which suggests that chemical castration can cut rates of reoffending to 5% from more than 40%. I'm trying to track down that evidence. The idea of chemical castration in the UK was first raised in 2007 by John Reid when he was home secretary, My colleague Alan Travis reported at the time:

Mr Reid also confirmed that he intends to step up voluntary drug treatment programmes, including the use of hormonal medication and antidepressant drugs, which suppress an offender's sexual urges. The home secretary strongly denied yesterday that such treatment programmes constituted "chemical castration" and said that as part of the package psychological treatment could be useful. Trials of such drugs have been going on in Britain for more than 20 years but the side-effects have so far proved too debilitating for the treatment to be effective.

Prof Don Grubin, of Newcastle University, has been running the three-year trial that was subsequently set up and which is reported on today. We'll be talking to him later.

The Mirror has some interesting "for and against" views about the programme, which sets out the issues well.

Dr Ludwig Lowenstein, psychologist, argues:

Chemical castration requires a high degree of supervision. Offenders have to keep taking the pills to lower their libido. Nor is it a total cure: paedophiles' sexual behaviour is governed not only by hormones but also by ­fantasies so they will still be drawn to children. The idea of giving sexual offenders a pill to destroy their ability to have intercourse always provokes fierce objections on the grounds of civil liberties. But a child's right to protection is far more morally important than the freedoms of paedophiles.

Frances Crook of the Howard League for Penal Reform

Sex offending is often not about sex at all, but about violence and domination. The drugs used will not affect those attitudes. Some men may inflict other types of deviant behaviour on victims if they are unable to perform sexually due to the drugs. The physical impact of the drugs can be very powerful, with the effect being similar to a sex change. "Feminising" sex offenders may make it more difficult for these individuals to reintegrate into society, which in turn makes their reoffending more likely.

I've just been speaking to Nick Pearce, director of the thinktank IPPR, (thanks to @joefd who suggested him via Twitter) who in 2006 made this film for Newsnight [video] about the Danish criminal justice system, including a focus on chemical castration. He said:

Scandanavian countries used to castrate people before WWII what they do now is that they essentially give people courses of treatment. You have to volunteer to do it. There is no force. You have to regularly be monitored. The drugs reduce testosterone, so reduces sex drive. Prisoners are released into the community and report regularly to outpatient clinic where they are tested regularly. They may commit other crimes but their repeat offending for sex offences is completely zero. They argue this is on the grounds that these drugs essentially reduce the urges they have and all the pain associated with things that make them paedophiles. It's small numbers in controlled conditions, for people who otherwise would never leave prison - violent sex offenders as well as paedophiles. It's not an alternative to therapeutic issues, but it takes out the urge that manifests itself in sexual violence. It takes away the thing that makes them violent because the testosterone is completely reduced. The evidence does show that repeat offending rate is low or zero. This is a voluntary treatment, it's not done as a populist punitive measure. The Danish justice system doesn't go for that. It's not an answer to sexual violence in society, it's a tiny number of cases. We have a very different culture of criminal justice. Denmark sees criminal justice as a collective responsibility to solve. In other states it might be used in much more punitive ways. The social context is important.

I'll update this blog shortly as I find more evidence, do get in touch with any relevant views of information.

I've just interviewed Professor Grubin who is in charge of the national programme for so-called "chemical castration".

It has been running for about three years and was set up after the then home secretary John Reid announced the plan in 2007. Prison psychologists or probation officers can refer prisoners to Grubin who then coordinates a local psychiatrist to treat them if, from the paperwork of their circumstances, he thinks it's appropriate. Overall about 100 patients have been referred to Grubin and he has arranged local treatment. The process is long and cumbersome and it's often difficult to find a psychiatrist willing and able to prescribe the drugs. There is no tracking or follow-up so Grubin can't say for sure that all of those 100 prisoners were subsequently treated, or that they stuck to the treatment. At Whatton, where 7-800 sex offenders are imprisoned, he set up a specialist clinic to provide more intensive and monitored care.

Grubin felt that the Mirror coverage was sensationalised, and not specific enough about the types of treatment being offered. Most prisoners who are referred to him – around 90% - would have been prescribed a Selective Serotonin Reuptake Inhibitor (SSRI) such as Prozac, which has the effect, he says, of "dampening" sexual desires. But it does not prevent men from having sex.

Other more severe cases, might be prescribed with an anti-androgen. In the UK this is typically cyproterone acetate or tryptorelin, not leuprorelin as cited by the Mirror, which is more often used in America. This is what is referred to as chemical castration, Grubin says, and in most cases takes away sexual desire, prevents a man's ability to get an erection or ejaculate.

He told me:

The difficulty can be when these guys have such high levels of sexual thinking is that they can't engage in psychiatric treatment. This allows them to. It isn't about making these people safe, or letting them out of prison and making it conditional. It's not being prescribed in that way. Typically they will have gone through a treatment programme and it's felt that they need more because of the strength of their fantasies. Depending on where they are they might then get started on the medication and then they would take part in a more in-depth treatment. The standard treatment is about getting people to recognise the problems in their behaviour, the triggers and managing that. What it doesn't do is focus on managing sexual arousal. Usually that's not the main problem. But for a minority of people they find it very difficult to control their urges. We're not talking about putting all rapist on this medication. Often these men don't want to be like this. For some their lives are really transformed because for the first time they aren't looking through sexualised spectacles at the world. We're saying there's a small group for whom it might help. We're not giving them an excuse, but recognising that this is one factor that might help them. Why for ideological reasons would you not give people something that might help? With sex offenders people want to be punishing them. But that's the job of judges. If you are looking at how can you control their risks, you might consider treatment. This isn't about punishment. You need to keep those things separate. If you want to lock them up all the time we don't need the discussion Anti-androgens have quite serious side effects which is why you would only use with volunteering and informed consent. They include osteoporosis. They mess up your blood lipids, so there can be coronary side effects. Many men find breast growth disturbing. It makes men into menopausal women, they get hot flushes, mood changes, redistribution of body fat. Men who stick with it they stick with it because it really can transform their lives.

Grubin says that there are no controlled trials of anti-androgens for sex offenders because of the ethical problems and because you couldn't use placebos because the side effects are so obvious.

There are a number of studies which say we've been prescribing anti androgens and there has been no re-offending. One Israeli study showed a marked impact on sexual behaviour. We know that they have a marked impact n sexual behaviour and arousal. The question is does that read over to reduction in reoffending?

Grubin makes two more controversial points that I'm mentioning separately as he has been misinterpreted on them before.

• He thinks in theory that that SSRIs work for some sex offenders with recurrent urges, because their behaviour has similar patterns to people with obsessive compulsive disorder (OCD), for which SSRIs are often prescribed. He wanted to make clear that this does not mean that people with OCD are similar to sex offenders in anyway or that having an OCD suggests that a person might also have inappropriate sexual urges.

Where it seems to be most effective is where men have recurrent urges. My feeling is these can be the same mechanisms as with OCD.

• He also raises the idea that men who have been on anti-androgens for some time, who request physical castration and have the capacity to understand what they are asking for, should be listened to.

Should we be refusing it if an individual has capacity, has been on medication and knows side effects? Even though it's a not a good decision if they have the capacity to make the decision should we be denying it? I think that's something to think about.

Below the line readers including @Loulu have been discussing the case of Alan Turing, the World War II code-breaker who in 1952 was prosecuted for homosexuality and treated with chemical castration as an alternative to prison. He later died in an apparent suicide. I've asked my colleague Martin Wainwright, who has written a lot about Turing, to write some more on this for us.

Martin Wainwright writes:

The computer genius Alan Turing is one of the best-known victims of chemical castration which he accepted as an alternative to prison after his conviction for gross indecency with another man in 1952. His injection with female hormones to reduce libido, a treatment then at an early, experimental stage, has become one of the most controversial aspects of an episode whose injustice and lack of sympathy attracts ever-increasing attention and disgust. Turing died aged 55 of cyanide poisoning in 1954, a tragedy assumed to have been suicide although never proved or officially recorded as such. The effects of his treatment were part of a miserable last stage of his life, when he also suffered exclusion from his brilliant, pioneering computer work and shaming publicity. His expertise was crucial to the cracking of the Nazis' wartime Enigma code and he was decorated for this, but the scale of its importance was still secret in 1954 and unknown to his Manchester university colleagues or the court. Although much-honoured posthumously and the subject of an official apology by the then Prime Minister, Gordon Brown, in 2009, hopes of an official pardon this year, the centenary of his birth, were dashed last month. Worldwide interest in the case is such that when the Guardian's Northerner blog ran a post on the pardon refusal last month, it was the most-viewed item on the Guardian website that week. The chemical castration issue featured prominently in the thread of comments to the piece.

Karen Ingala Smith, chief executive of Nia, which campaigns against violence against women and children, says:

One of the most important things, if we want to reduce violence and abuse, is for a perpetrator to take responsibility for what they have done. The issue is not only that someone feels an urge to do something but that they realise that it is wrong and take steps to control or curb their behaviour. If volunteering to take drugs is helpful, and there appears to be evidence to suggest that it is, than we would welcome this as part of a range of interventions. However, drugs that control testosterone, will not change attitudes that condone violence and abuse. There is no single thing that causes sexual violence and there is no single cure. If we are serious about reducing sexual abuse and violence we need to combine a range of strategies including widespread prevention work so that everyone knows what a healthy relationship is and how notions of masculinity can be damaging for men as well as women, boys as well as girls. This should be backed up by intensive sex offender treatment through rehabilitation programmes in prison, effective criminal justice responses including police and public protection on release and of course well-resourced victim support services.

Frances Flaxington, who worked in probation for 15 years (including with sex offenders), was deputy chief inspector of probation and is now lead for the charity C22 on offender rehabilitation in the community, has got in touch via Twitter. She suggests that a study on the use of polygraphs (lie-detectors) to track the behaviour of sex offenders is about to be published that will show an interesting alternative to chemical castration. She said:

I used to work with sex offenders in probation. I always felt that if it was about power then chemical castration wouldn't tackle the issue. It sounds dramatic and like it will reduce libido and solve the problem but it depends on the underlying reason for that behaviour. What drives sex offenders is really complex. It's not just about sex. They have different motivations. You need to think about every option at your disposal and tailor programmes for individual prisoners.

The evidence

I've been looking at this Israeli study that Grubin mentioned above. It followed 30 men with paraphilia – severe deviant sexual behaviour – through a longterm course of anti-androgens and found a dramatic reduction in deviant behaviour and fantasies or desires. The Israeli study also states that such chemical "castration" using anti-androgens is reversible. It says:

All the men had a decrease in the number of deviant sexual fantasies and desires, from a mean (±SD) of 48±10 per week before therapy to zero during therapy (P<0.001), and a decrease in the number of incidents of abnormal sexual behavior (from 5±2 per month to zero, P<0.001) while receiving triptorelin. These effects were evident after 3 to 10 months of therapy (P<0.001) and persisted in all 24 men who continued therapy for at least 1 year. Continuous administration of triptorelin, a long-acting agonist analogue of gonadotropin-releasing hormone, together with supportive psychotherapy, may be an effective treatment for men with severe paraphilia.

This study looked at the use of SSRI anti-depressants to treat sex offenders with sexual compulsions, as opposed to paraphilia, suggesting that these can be effective, especially where anti-androgens are not appropriate.

This Korean review of the international evidence to date finds that there is strong evidence that chemical castration prevents repeat sexual offending, but the finding is qualified by the fact that there are no trials with placebos. It says:

In the past, physical castration was adopted to punish sex offenders and prevent the recurrence of sexual crimes. However, it was abrogated because of human right issues and the irreversibility of fertility. Chemical castration of depot injection with hormones was introduced as an alternative method of physical castration. Antiandrogen is the most frequently used hormonal agents. Although there are several positive results such as changes of sexual behaviors and decrease of the recurrence rate of sexual crime after hormonal treatment, it also has serious limitations; difficulties in academic evaluation for control group and placebo effect; difficulties in the assessment of the therapeutic effect; and difficulties to decide the proper duration of treatment.

The Guardian's science correspondent, Ian Sample, has been looking at the evidence as well. He writes:

Writing in the Prison Service Journal in 2008, Karen Harrison, a senior law lecturer at the University of the West of England, outlined some of the issues in using the antiandrogen drug, cyproterone acetate, to reduce re-offending rates in sex offenders. Although the drug usually led to a drop in sexual desire, the effective dose varied enormously, from as little as 10mg per week to 200mg per day. Some physicians have raised concerns over the side effects of another drug, medroxyprogesterone acetate, which is licensed in the US. People who take the drug can suffer migraines, gallstones, blood clots, allergic reactions and even suicidal thoughts. Chemical castration does appear to work, especially when it is used alongside psychological counselling. In one Danish study, there were no reoffences by 17 men who had continuous treatment from 1989 to 1997. Of five men who discontinued treatment, one reoffended in the five years after.

Below the line, PaulusMaximus writes:

I'd look to Germany for info on recidivism - they have a voluntary scheme where sex offenders are given the option of physical castration. For those who go through with the scheme recidivism is much lower, of the 104 who underwent the procedure between 1970 and 1980 there was a 3% rate of reoffending, compared to almost 50% of those who refused the option or were denied it.

Headlines stating that "100 paedophiles are chemically castrated in jail" are stretching the truth. Prof Grubin, the head of the UK programme, told us that 100 imprisoned sex offenders have been referred to him but the vast majority of those, Grubin says around 90%, will have been treated using SSRI anti-depressants, which are believed to dampen sexually compulsive behaviour. This cannot be described as chemical castration because patients receiving this treatment are still able to have sex. A small number have been referred for anti-androgen treatment which leaves men with pre-pubescent testosterone levels similar to those in men who have been physically castrated. This so-called chemical castration is reversible and therefore some people argue that the phrase "castration", which suggests permanence, is not accurate.

Small scale studies suggest that both these methods reduce sexually deviant thought, behaviour and reoffending rates. However, because it's impossible to conduct full trials of the drug using placebos, these results are not conclusive. There are also serious side-effects of the anti-androgen treatment including osteoporosis.

In the UK participation in the programme is on an entirely voluntary basis. There is no suggestion that it would be made compulsory. However, some psychologists argue that prisoners who are trying to get parole may feel compelled to undergo such treatments.

In Denmark the programme is seen as a part of the rehabilitation programme but Nick Pearce of the IPPR acknowledged that in a UK context where there is populist pressure for harsher punishments for certain crimes, it could be seen as a punititive instead.

Victim support groups, probation experts and penal reform charities all point out that sexual violence is not just about sex, but power and control. To treat sex in isolation will not solve the underlying problem. Frances Crook of the Howard League for Penal Reform said:

Sex offending is often not about sex at all, but about violence and domination. The drugs used will not affect those attitudes. Some men may inflict other types of deviant behaviour on victims if they are unable to perform sexually due to the drugs. The physical impact of the drugs can be very powerful, with the effect being similar to a sex change.

Update

The Ministry of Justice got back to me last night after I asked them to provide the evidence for their decision to back the chemical "castration" programme. They said these guidelines for the biological treatment of paraphilias, from The World Federation of Societies of Biological Psychiatry (WFSBP), were their basis.