New studies on the effect of grapefruit juice on medication has found it can interact with many drugs, sometimes with dangerous consequences.

Drinking the juice with medication can change the way the drug works in the body.

Researchers have learned that the fruit temporarily knocks out an enzyme in the intestine, an effect that can last for 24 hours after consumption. Certain drugs get into the body through this enzyme.

Once grapefruit is added to the mix, the drug isn't metabolized and seeps through the gut and into the bloodstream.

"You don't have to worry about that with most drugs. Most drugs have a big margin of safety," says researcher David Spence.

"You could take ten times the dose of penicillin and it would be very unlikely to cause trouble. But for some drugs, there's a narrow margin of safety. A narrow difference between an effective dose and a toxic dose."

Can affect blood pressure, kidneys

Those drugs include ones that lower blood pressure. Blended with grapefruit jucie, the drugs can force your blood pressure to go too low. The heart beats too quickly, you might feel weak and even collapse.

In severe cases, grapefruit, in combination with drugs used to lower cholesterol, can lead to kidney problems.

The allergy medication, Seldane, combined with grapefruit juice, is especially dangerous. At least one death is blamed on the combination. Seldane is no longer available in Canada.

The known list of drugs that interact with grapefruit juice is growing. Pharmacists are instructed to warn their clients of the dangers. Spence says that's not enough.

"I think there needs to be labelling of grapefruit and grapefruit juice saying check... with your pharmacist or check with your doctor before you take this food substance if you're on medications."

Spence says people should be alarmed by the "grapefruit effect," and he wishes the federal government would take it more seriously.

It's not just grapefruit. There's evidence the Valencia oranges may have a more modest effect. And research continues on other products such as red wine, tea, corn and seaweed.

There is an up side to grapefruit juice: besides providing vitamin C, it can make some drugs more potent.

"One of the AIDs drugs, called saquinavir, has routinely been given with grapefruit juice because it's expensive and when you take it with grapefruit you don't have to use as much," says Spence.