Scientists are seeking crowdfunding to test 'chemical castration' of paedophiles, with a hormone therapy that blocks brain signals and reduces urges.

Researchers from Sweden are seeking crowdfunding to test a type of "chemical castration" in men who report having paedophilic thoughts and fantasies.

The team from Sweden's Karolinska Institute want to see whether a drug called degarelix - a hormone therapy that blocks brain signals which stimulate the testicles to produce testosterone - reduces the men's sexual urges.

Experts estimate that up to five per cent of the general population has paedophilia, a disorder marked by persistent sexual attraction to pre- or early-pubescent children.

While not all people with paedophilia molest children, child sexual abuse is a widespread problem with around one in 10 girls and one in 20 boys suffering abuse, according to Christoffer Rahm, a Swedish consultant psychiatrist leading the planned trial.

Experts say testosterone is key to at least three risk factors for abusing children sexually - high sexual arousal, disturbed self-regulation and low empathy - so lowering testosterone could reduce the risk of men with paedophilic traits molesting children.

Rahm said "a substantial number of patients with paedophilic disorder actually want help", but don't know how or where to get it.

A key problem, he added, is that there are no evidence-based preventive treatments and no reliable risk assessment tools.

"There is an urgent need for more research to be done," he told reporters at a briefing in London.

Rahm said the majority of research into paedophilia has tended to be reactive - beginning "when the harm has already been done" and focussing on stopping perpetrators committing more acts of abuse.

"I want to shift the focus and explore methods of preventing child sexual abuse from happening in the first place," he said.

Degarelix is a rapidly acting injectable drug that can reduce testosterone to undetectable levels within three days.

Researchers hope it may have measurable risk-reducing effects within two weeks, which could last for up to three months.

Rahm's team is working with the UK-based science crowdfunding platform Walacea and asking the public for a total of 38,000 pounds ($A70,423) to conduct the research.

If the funding goal is met, the study will run from 2016 to 2018. It will include around 60 men and will be a double-blind randomised study - meaning participants will be randomly given either the drug or a placebo, and neither they nor the researchers will know at the time whether they are getting the real or the placebo injection.