The researchers from the Massachusetts Institute of Technology say that phentermine, a popular anti-obesity treatment, could damage patients' heart valves and lungs when taken with anti-depressants such as Prozac. The two drugs are often prescribed together in the US to patients attempting to lose weight.
The two companies vigorously rejected the charges, saying that the MIT lecturers' findings were based on rat studies from the 1970s which, in fact, explicitly contradict the authors' hypotheses.
Medeva also made counter allegations that one of the researchers had a conflict of interest as he was the co-inventor of a rival diet drug.
Drs Timothy Maher and Richard Wurtman, the two researchers, make their allegations in a paper on Wednesday to the International Congress on Obesity, in Paris .
In the study, abstracts of which have been seen by The Independent, the two argue that the combination of phentermine, which is not sold in Britain, and Prozac could destroy the ability of the body to control serotonin in the blood.
Too much serotonin damages blood vessels, particularly in the lungs, and can harm heart valves. However, they admit that nobody has been diagnosed as having developed heart and lung diseases as a result of taking the drugs.
SmithKline and Medeva dismissed the allegations and said the diet drug was safe.