A range of drugs widely used to treat high blood pressure and heart failure may be linked to cancer, study results suggest.
Use of angiotensin receptor blockers (ARB) was associated with a "modestly increased risk" of new cancer diagnosis, said researchers.
Although the likelihood of actually developing cancer while taking one of the drugs was small, one expert called the findings "disturbing and provocative".
Others urged patients on the potentially life-saving drugs not to stop taking their medications.
ARBs are commonly prescribed for the treatment of high blood pressure, heart failure, and kidney damage due to diabetes.
They work by blocking the activity of a hormone that prevents blood vessels from dilating, thus raising blood pressure.
Although the drugs are considered safe, one previous trial reported a significant increase in fatal cancers in patients treated with the ARB candesartan.
To carry out the new investigation, US researchers pooled together and analysed all the previously published data from ARB studies prior to November 2009.
They also looked at new data from five trials involving 61,590 patients, and trends relating to lung, prostate and breast cancers, and cancer deaths. Part of the analysis involved deaths among 93,515 patients who took part in eight trials.
More than 85% of the patients were taking one type of ARB, telmisartan.
The findings showed that 7.2% of patients taking ARBs were diagnosed with a new cancer over a period of four years compared with 6% of patients not treated with the drugs.
Although the result was significant, the authors led by Dr Ilke Sipahi, from Case Western Reserve University School of Medicine in Cleveland, Ohio, stressed that it should be seen in context.
"The finding of a 1.2% increase in absolute cancer risk over an average of four years needs to be interpreted in view of the estimated 41% background lifetime cancer risk," they wrote in The Lancet Oncology medical journal.
Among specific solid organ cancers examined, only the risk of lung cancer was significantly increased in patients taking ARBs. Their risk was raised from 0.7% to 0.9%.
No significant excess in cancer deaths was found, although the researchers said this may only have been because of the short follow-up time.
In an accompanying comment article, Dr Steven Nissen, from the Cleveland Clinic in Cleveland, said: "The meta-analysis... by Sipahi and colleagues is disturbing and provocative, raising crucial drug safety questions for practitioners and the regulatory community."
But two British experts said the research should be viewed with caution.
Martin Ledwick, head information nurse at Cancer Research UK, said: "At the moment there isn't enough evidence to draw any firm conclusions about how blood pressure drugs might affect cancer risk and this will need further investigation.
"People shouldn't stop taking these drugs on the basis of this research, if they're concerned they should speak to their GP."
Dr Tim Chico, from the Medical Research Council Centre for Developmental and Biomedical Genetics at the University of Sheffield, said: "This study does not prove that these drugs cause cancer, although it is certain that lowering blood pressure save lives by preventing strokes and heart attacks. If people taking medication are worried about possible side effects, I recommend they discuss this with their GP."
Boehringer Ingelheim manufactures the angiotensin receptor blocker (ARB) telmisartan under the brand name Micardis.
The company's corporate senior vice president medicine Professor Dr Klaus Dugi said: "Our research efforts have centred on the need to protect patients, especially older patients from cardiovascular risks such as myocardial infarction or stroke. Telmisartan fulfils this need. It is the only ARB which has cardiovascular protection in its label and has become a valuable treatment option in the management of hypertension and cardiovascular risk.
"Doctors and patients appreciate its excellent safety profile.
"In pre-clinical trials, clinical trials and day-to-day patient exposure with telmisartan, we have not seen any significant finding related to malignancies. Patients should consult with their physicians before making any decision regarding their antihypertensive therapy."