The fruit contains chemicals that can interact with certain drugs, making them more potent.
Adverse effects can include acute kidney failure, respiratory failure, internal bleeding and sudden death.
While it is well known that some patients should avoid grapefruit, or grapefruit juice, the list of drugs involved has risen sharply in recent years.
Between 2008 and 2012, the number of medications with the potential to cause serious harm by interacting with grapefruit increased from 17 to 43, said scientists writing in the Canadian Medical Association Journal.
Dr David Bailey, from Lawson Health Research Institute in London, Ontario, wrote: "Unless healthcare professionals are aware of the possibility that the adverse event they are seeing might have an origin in the recent addition of grapefruit to the patient's diet, it is very unlikely that they will investigate it."