Women could have been told about the risks of taking hormone replacement therapy years ago.
HRT can help many women going through the menopause
Researchers say studies showing HRT may increase the risk of heart disease were published as early as 1997.
But most were done by drug companies and were not publicly available. The risk was only formally acknowledged in 2002 when a large US trial was halted.
Writing in the British Medical Journal, the researchers called for drug firms to be more open about their research.
Professor Klim McPherson of Oxford University and Professor Elina Hemminki of the National Research and Development Centre for Welfare and Health in Helsinki reviewed 23 studies, published before 2002.
That was the year the Women's Health Initiative trial was halted. The study, which involved 16,000 women, was stopped when scientists realised that women on HRT had a higher risk of heart disease and breast cancer compared to those taking dummy drugs.
Many of the studies they looked at were done by pharmaceutical companies and were used to help them get licences for HRT drugs.
They found that women on HRT were more likely to have heart problems.
"A higher proportion of the women taking hormone replacement therapy had cardiovascular events than women in the control groups," they said.
The findings were published in 1997 and were widely rejected. "When we published these findings in 1997, we were ridiculed," they said.
Writing in the BMJ this week, they said data from six other trials had shown similar findings. However, they said they had to go through the courts to obtain access to these studies, which had been carried out by drugs companies.
The researchers called for new rules to ensure their is better access to studies done by the pharmaceutical industry.
"Such studies should be in registers of clinical trials and when legitimate anxiety about the safety of the products is raised, the data from all such studies should be made available to independent scientists and regulators.
"This is not a great deal to ask," they said. "How long will it take us to learn? How many women were needlessly exposed to an increased risk of cardiovascular disease?"
The Association of the British Pharmaceutical Industry said drug companies are becoming more open with their research.
"We have set up a one stop shop online listing clinical trial data - the Pharmaceutical Industry Clinical Trials database," said a spokesman.
"A lot of companies have signed up although some have not. We are encouraging more to join.
"It gives summary information and details on where to go to obtain more information on studies."