There is no conclusive evidence that sex change operations improve the lives of transsexuals, with many people remaining severely distressed and even suicidal after the operation, according to a medical review conducted exclusively for Guardian Weekend tomorrow.

The review of more than 100 international medical studies of post-operative transsexuals by the University of Birmingham's aggressive research intelligence facility (Arif) found no robust scientific evidence that gender reassignment surgery is clinically effective.

The Guardian asked Arif to conduct the review after speaking to several people who regret changing gender or believe that the medical care they received failed to prepare them for their new lives. They explain why they are unhappy with their sex change and how they cope with the consequences in the Weekend magazine tomorrow (July 31).

Chris Hyde, the director of Arif, said: "There is a huge uncertainty over whether changing someone's sex is a good or a bad thing. While no doubt great care is taken to ensure that appropriate patients undergo gender reassignment, there's still a large number of people who have the surgery but remain traumatised - often to the point of committing suicide."

Arif, which advises the NHS in the West Midlands about the evidence base of healthcare treatments, found that most of the medical research on gender reassignment was poorly designed, which skewed the results to suggest that sex change operations are beneficial.

Its review warns that the results of many gender reassignment studies are unsound because researchers lost track of more than half of the participants. For example, in a five-year study of 727 post-operative transsexuals published last year, 495 people dropped out for unknown reasons. Dr Hyde said the high drop out rate could reflect high levels of dissatisfaction or even suicide among post-operative transsexuals. He called for the causes of their deaths to be tracked to provide more evidence.

Dr Hyde said: "The bottom line is that although it's clear that some people do well with gender reassignment surgery, the available research does little to reassure about how many patients do badly and, if so, how badly."

There are around 5,000 post-operative transsexuals in the UK, according to the transgender pressure group Press for Change (PFC). It is estimated that up to 400 sex changes will be performed this year on the NHS and privately. Each operation costs the NHS around £3,000, while private patients pay upwards of £8,000 for surgery.

Christine Burns, of PFC, said the campaign group's research suggested that the vast majority of transsexual people enjoyed much happier lives following surgery.

Ms Burns added that the greatest flaws in medical literature about gender reassignment were in those studies unsympathetic to transsexual people. For example, one study was based on a survey of seven transsexual prostitutes interviewed in one gay bar in Chicago.

She said: "The fact that research is badly constructed isn't a poor reflection on transpeople, but on the people we should be able to trust for our care. If they "lose" half the patients they ought to be able to track the question is why? As we've repeatedly pointed out ourselves there is really no difficulty in getting transpeople to come forward and cooperate in research that is properly constructed and conceived with people's true well-being in mind."

Research from the US and Holland suggests that up to a fifth of patients regret changing sex. A 1998 review by the Research and Development Directorate of the NHS Executive found attempted suicide rates of up to 18% noted in some medical studies of gender reassignment.

Andrew McCulloch, chief executive of the Mental Health Foundation, has written to the mental health minister, Rosie Winterton, requesting a "thorough assessment" of the long-term effects of sex change operations. He wants the National Institute for Clinical Excellence, which decides what treatments should be available on the NHS, to draw up guidelines on gender reassignment.

Transgender psychiatrists, who assess whether patients should change sex, agree that more scientific research is needed. But Kevan Wylie, chairman of the Royal College of Psychiatrists' working party on gender identity disorders, said that all of his patients' lives have drastically improved following gender reassignment surgery.

Dr Wylie added that it was difficult to conduct research on the outcome of gender reassignment, or to compare its effects with alternative treatments, because transsexualism was such a "rare experience". Urological surgeon James Bellringer, who has performed more than 200 sex changes over the past four years, claimed that trying to carry out research that involves studying a control group of transsexual patients who were denied hormones and surgery would be unethical.

Mr Bellringer, who works at the main NHS gender identity clinic at Charing Cross hospital in west London, said: "I don't think that any research that denied transsexual patients treatment would get past an ethics committee. There's no other treatment that works. You either have an operation or suffer a miserable life. A fifth of those who don't get treatment commit suicide."