A class of drugs commonly used to treat heart problems has been linked with a "modestly" increased risk of cancer.
Analysis of published data from all trials of angiotensin-receptor blockers (ARBs) found one extra case of cancer for every 105 patients treated.
The US researchers said the evidence from nine trials should prompt drug regulators to investigate.
But they advised people not to stop taking the drugs, but to see their doctors if concerned.
The results are published in The Lancet Oncology.
ARBs are mainly prescribed for conditions such as high blood pressure and heart failure.
They are used by millions of people worldwide.
In 2003, a trial of one of the drugs found an increased risk of death from cancer but it was unclear if that was a significant finding.
In the latest analysis, researchers looked at all publicly available data and found information on lung, prostate and breast cancer for 68,000 patients, new cancer diagnoses for 61,000 patients, and cancer deaths for 93,000 patients.
Overall, they found that taking ARBs was associated with a significantly increased likelihood of a new cancer diagnosis compared with taking a dummy pill or other heart medications (7.2% vs 6.0%).
When looking at specific types of cancer, they found that only lung cancer was significantly higher in those taking ARBs.
And there was no real difference in cancer deaths although the researchers point out that may be because patients were not assessed for long enough.
It is not clear why ARBs may increase cancer risk but some animal studies suggest there may be a link with blood vessel growth in tumours.
Although the increased risk found is a modest one, study leader Dr Ilke Sipahi, from The University Hospitals Case Medical Center in Cleveland, said because the drugs are so widely used, it potentially adds up to a lot of extra cancers.
He said before anyone acts on the findings, drug regulators, who have access to far more detailed information, need to look closely at factors such as age, smoking history and duration of use to clarify the risk.
More calculation is also needed on whether some ARBs are linked with a higher risk than others, he added.
"My message for the patients who are currently on ARBs is they should not stop taking these medications on their own because they have beneficial effects.
"They need to make a decision with their physicians about the potential risks and whether they need to switch."
Judy O'Sullivan, senior cardiac nurse at the British Heart Foundation, agreed that further investigation was needed before any conclusions could be drawn.
"The benefits of taking the drug are well established and it remains an effective treatment for many of the 2.6 million people in the UK living with coronary heart disease."
Dr Tim Chico, deputy director of the NIHR Cardiovascular Biomedical Research Unit at the University of Sheffield, stressed that the study did not prove a link but that drugs like ARBs which lower blood pressure save lives.
"If people taking medication are worried about possible side effects, I recommend they discuss this with their GP."