Why does dairy make antibiotics less effective?

You may have heard that some foods and drinks, such as grapefruit and alcohol, shouldn't be consumed with certain medications. But did you know that dairy products can make some drugs less effective?

This effect is particularly problematic for certain classes of antibiotics, which are drugs used to treat bacterial infections. Antibiotics taken by mouth — in a pill or liquid form, for example — need to be absorbed into the bloodstream to work effectively. But when taken with dairy products, such as milk, cheese or yogurt, some antibiotics are not absorbed as they should be.

For instance, one study found that the absorption of an antibiotic called demeclocycline (brand name Declomycin) into the bloodstream dropped by a staggering 83% when taken with milk, compared with water and a dairy-free meal. Demeclocycline is used to treat bacterial infections of the respiratory tract, like pneumonia. Another study showed that when ciprofloxacin is taken with milk, levels of the drug in the bloodstream are about 30% to 36% lower than when it is taken with water. This antibiotic is used to treat a range of illnesses, from pneumonia to gonorrhea.

There is less data on the effects of other dairy products, but studies generally suggest that they also lower antibiotic levels in the blood, although not necessarily as much as a glass of milk would.

But how does dairy hinder the absorption of these drugs?

Related: Why do leafy green vegetables interact with blood thinners?

Dairy products contain an abundance of charged calcium and magnesium particles, called "ions." These ions can bind to some antibiotics, creating "an insoluble compound," Anastasiya Shor, an assistant professor at Touro College of Pharmacy in New York, told Live Science in an email. Insoluble compounds do not dissolve in water, and if a drug cannot dissolve in the fluid inside the gut, it cannot be absorbed into the bloodstream.

So, when calcium and magnesium ions are present in the gut, they effectively trap the antibiotic molecules in there. The amount of antibiotic that makes it into the bloodstream is thus significantly reduced, diminishing the drug's effectiveness throughout the body.

This interaction with dairy products is an issue only with certain antibiotics, specifically ones that belong to two classes: tetracyclines and fluoroquinolones. It doesn't affect all antibiotics.

And fortunately, it's fairly simple to avoid this drug interaction. The key is to take antibiotics with water and to time your meals and medications.

"To make sure the antibiotic is working as intended, patients should take the drug 2 hours before or 6 hours after consuming dairy products," Shor said. "This reduces the contact of the calcium and the antibiotic in the digestive tract, and ensures the patient is getting the maximum benefit from the medication."

It's also important to be cautious about other, nondairy sources of calcium or magnesium, such as antacids (heartburn medication) and dietary supplements. Just like milk products, these substances can hinder the effectiveness of some antibiotics, Shor cautioned.

And it isn't just antibiotics that interact with the calcium in dairy products; other drugs can also be impacted.

A study published in the European Journal of Pharmacology found that, when the prostate cancer drug estramustine (brand name Emcyt) is taken with milk, it forms a complex with calcium that makes it harder for the body to absorb the medication. Another study, published in the journal Acta Pharmacologica et Toxicologica, found that the drug sotalol (brand name Betapace), which is used to treat irregular heartbeat, also binds with calcium. This bond reduces the drug's absorption and effectiveness when it's taken with milk.

Sometimes, it's not the calcium, but rather a milk protein called casein, that's to blame. A study published in the journal Die Pharmazie found that casein can form complexes with drugs like phenytoin, a medication used to control seizures, and thus reduce the absorption of the drug in the digestive tract.

Enzymes in milk can also cause problems. According to a study in the Journal of Oncology Pharmacy Practice, the cancer drug mercaptopurine is inactivated by an enzyme called xanthine oxidase, which is found in high concentrations in milk. To avoid this interaction, patients taking mercaptopurine are advised to space out their milk intake from the time they take the medication to ensure the drug works as intended.

To avoid any possible food-drug interactions with dairy, it's important to consult the pharmacist when picking up medications, Shor said. A medical provider can help clarify if dairy or other sources of calcium or magnesium would make a given medication less potent, and potential drug interactions are also typically listed on drug labels.

This article is for informational purposes only and is not meant to offer medical advice.

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