Oklahoma may soon be added to the list of states that give the option of chemical castration as punishment for sex offenders.

However, even in a conservative state with a tough-on-crime reputation, the legislation is likely to face strong opposition.

Rick West, a first-term Oklahoma state representative, filed the bill at the request of a constituent and says he fully intends to push for its passage.

Under West's bill, anyone convicted of a sexually violent offence could be required as a condition of release to take the drugs designed to reduce a male offender's testosterone and sexual libido. A second offence would require the treatment unless a court determined it would not be effective.

At least seven other US states have laws allowing courts to order chemical treatments that reduce male testosterone for certain sex offenders. Experts say the punishment is rarely carried out, with one describing it as a "half fantasy" version of criminal justice.

"When I knocked on that guy's door when I was campaigning, he said: 'I'll vote for you if you'll run this bill,'" West said, explaining he is confident his constituents would support efforts to prevent sex crimes, especially against children.

California became the first state to pass a chemical castration law in 1996, and since then at least six other states have passed laws allowing it in some form, including Georgia, Florida, Louisiana, Montana, Oregon and Wisconsin, according to the National Conference of State Legislatures. Texas allows repeat sex offenders to voluntarily elect to be surgically castrated.

It is unclear how often the procedure is used, but it appears to be rare. For it to be used in California, a judge would have to issue an order as part of a convict's sentence. Only a couple of parolees are currently required to receive the treatment every year, said prisons spokesman Luis Patino.

Prison officials in Montana and Louisiana are aware of only one case in each state in the last decade in which a judge ordered the treatment.

'Half advertising slogan, half fantasy'

Oklahoma's American Civil Liberties Union chapter is concerned about West's proposal, saying that requiring unwilling offenders to undergo such treatments likely violates the Constitution's 8th Amendment.

"It's hard to imagine this couldn't be considered cruel or unusual," said chapter spokeswoman Allie Shinn, who added there is little scientific evidence to suggest such treatments are even effective.

"I don't want to place too much faith in the Oklahoma legislature to avoid blatantly unconstitutional proposals, but we're hopeful this bill, as written, is just too extreme to move," Shinn said.

While drugs used to diminish an offender's sex drive can be effective, they are mostly successful with offenders who want to change their behaviour and take them as prescribed, said Frank Zimring, a law professor at University of California at Berkeley and an expert on sex crimes.

But he said the laws are generally about good politics since sex offenders are an easy target, and not necessarily about sound criminal justice policy.

"Chemical castration is half advertising slogan, half fantasy," Zimring said. "There are chemicals which are supposed to, if dosages are maintained, reduce sex drives. That isn't castration."

The Oklahoma Legislature has, over the years, entertained various bills involving the castration of sex offenders. In 2002, a measure allowing chemical or surgical castration of sex offenders made it all the way to the desk of Republican governor Frank Keating, who promptly vetoed it and derided it as "silly".