Many medications, such as statins and some antihistamines, have a negative interaction with grapefruit. Metformin is used in treatment of type 2 diabetes.
Does having grapefruit while taking metformin lead to adverse side effects? There’s limited research, but here’s what you need to know.
What is metformin?
Metformin is a drug that’s prescribed to treat type 2 diabetes. People with type 2 diabetes can’t use insulin normally. This means they can’t control the amount of sugar in their blood. Metformin helps people with type 2 diabetes control the level of sugar in their blood in several ways, including:
- decreasing the amount of sugar your body absorbs from food
- decreasing the amount of sugar produced by your liver
- increasing your body’s response to the insulin that it makes naturally
Metformin can rarely cause a very serious and life-threatening condition called lactic acidosis. People with liver, kidney, or heart problems should avoid taking metformin.
How drug interactions with grapefruit work
There are more than 85 drugs that are known to interact with grapefruit. Of these drugs, 43 of them can lead to serious adverse effects. All forms of grapefruit — including freshly squeezed juice, frozen concentrate, and the whole fruit — can lead to drug interaction.
Some of the chemicals found in grapefruit can bind to and inactivate an enzyme in your body that’s found in your intestines and liver. This enzyme helps break down the medication you take.
Normally when you take a drug orally, it’s broken down slightly by enzymes before it reaches your bloodstream. This means that you receive a little less of the drug in your bloodstream than the amount you initially consumed.
But when the enzyme is inhibited — as it is when it interacts with the chemicals in grapefruit — there’s a dramatically larger amount of the drug that makes its way into your bloodstream. This leads to a higher risk of overdose. Take a more in-depth look at grapefruit-drug interactions.
What drugs interact with grapefruit?
According to the U.S. Food and Drug Administration (FDA), the following types of drugs can have a negative interaction with grapefruit:
- statins, such as simvastatin (Zocor) and atorvastatin (Lipitor)
- drugs for high blood pressure, such as nifedipine (Procardia)
- immunosuppressive drugs, such as cyclosporine (Sandimmune)
- corticosteroids used to treat Crohn’s disease or ulcerative colitis, such as budesonide (Entocort EC)
- drugs that treat abnormal heart rhythms, such as amiodarone (Pacerone)
- antihistamines, such as fexofenadine (Allegra)
- some anti-anxiety drugs, such as buspirone (BuSpar)
Grapefruit juice doesn’t have an effect on every drug in the categories above. Interaction with grapefruit juice is drug-specific, not drug category-specific.
When starting on a new medication, it’s very important that you ask your doctor or pharmacist if you’re able to consume grapefruit or grapefruit-related products.
How does grapefruit affect metformin?
It’s important to know that metformin isn’t broken down by the same enzyme as the drugs listed above. It’s unprocessed by your body and expelled in your urine.
There’s limited information available as to how having grapefruit while taking metformin affects people with type 2 diabetes.
A 2009 paper discussed the effects of grapefruit with metformin in nondiabetic rats. Some rats were exposed to grapefruit juice and metformin. Others were exposed to metformin alone. Researchers found that there was an increase in the amount of lactic acid production in the rats that were exposed to grapefruit juice and metformin.
Researchers guessed that grapefruit juice enhanced metformin accumulation in the liver. This, in turn, caused the increase in lactic acid production. Because of this, the researchers suggested that drinking grapefruit juice may lead to an increased risk of lactic acidosis in people taking metformin.
However, these results were observed in nondiabetic rats, not in humans with type 2 diabetes. To date, there hasn’t been a case study in humans that indicates that taking metformin with grapefruit juice leads to lactic acidosis.
Other things to avoid while on metformin
Taking some medications while taking metformin can increase the risk of developing lactic acidosis. You should let your doctor know if you’re taking any of the following medications:
- diuretics, such as acetazolamide
- corticosteroids, such as prednisone
- blood pressure medication, such as amlodipine (Norvasc)
- anticonvulsants, such as topiramate (Topamax) and zonisamide (Zonegran)
- oral contraceptives
- antipsychotic drugs, such as chlorpromazine
Avoid consuming large amounts of alcohol while on metformin. Drinking alcohol while taking metformin increases your risk of developing low blood sugar or even lactic acidosis.
According to the University of Michigan, you should avoid eating high-fiber foods after taking metformin. This is because fiber can bind to drugs and lower their concentration. Metformin levels decrease when taken with large amounts of fiber (greater than 30 milligrams per day).
Some general diet guidelines for people with diabetes are as follows:
- Include carbohydrates that come from vegetables, fruits, and whole grains. Be sure to monitor your carbohydrate intake, as this will directly affect your blood sugar.
- Avoid food that’s high in saturated and trans fats. Instead, consume fats from fish, nuts, and olive oil. Here are 10 ways to add healthy fats to your diet.
- Eating 25 to 30 milligrams of fiber per day may help control blood glucose levels. See this list of 22 high-fiber foods to get started.
- Avoid sodium. Try to consume less than 2,300 milligrams per day.
How grapefruit can help people with diabetes
Drinking grapefruit juice may actually be beneficial if you have diabetes.
An animal study showed that drinking preparations of clarified grapefruit juice reduced both fasting glucose and weight gain. The effects observed were similar to the effects of metformin. There was no enhanced effect when grapefruit juice and metformin were tested together.
While promising, it’s important to note that these observations were made in a mouse model of diabetes.
A review of the role of grapefruit in diet and drug interaction also suggests grapefruit is associated with weight loss and improved insulin resistance. What’s more, the review also reports a compound in grapefruit juice (naringin) has been found to improve hyperglycemia and high cholesterol in a type 2 diabetes animal model. Learn more about living with diabetes and high cholesterol.
Grapefruit does lead to negative interactions with some medications. However, there are no case studies in which consuming grapefruit juice while taking metformin led to adverse effects in humans.
There’s some promising experimental evidence that including grapefruit in your diet can help promote weight loss and reduce fasting glucose levels.
If you’re taking metformin and are concerned about drug-drug interactions or food-drug interactions, talk to your healthcare provider.