Why does milk reduce the effectiveness of antibiotics?

Oral antibiotics — such as pills or liquid — need to be absorbed into the bloodstream to work effectively. However, when taken with dairy products such as milk, cheese or yogurt, some antibiotics are not absorbed as expected.

According to research, the absorption of an antibiotic called demeclocycline (named Declomycin) into the bloodstream is reduced by up to 83% when taken with milk compared to when taken with water and a meal without milk. Demeclocycline is used to treat respiratory infections, such as pneumonia.

A study published in the journal European Journal of Pharmacology discovered that when the prostate cancer treatment drug estramustine (named Emcyt) is taken with milk, it forms a complex with calcium, making it more difficult for the body to absorb the drug.

Another study, published in the journal Acta Pharmacologica et Toxicologicadiscovered that the drug sotalol (named Betapace), used to treat irregular heartbeat, also binds to calcium. This combination reduces the absorption and effectiveness of the drug when taken with milk.

Another study found that when ciprofloxacin was taken with milk, drug levels in the blood were reduced by about 30% to 36% compared to when taken with water. This antibiotic is used to treat a variety of diseases, from pneumonia to gonorrhea.

There is less data on the effects of other dairy products, but studies generally show that they also reduce blood levels of antibiotics.

Scientists explain that milk interferes with the absorption of these drugs because it contains a lot of calcium and magnesium ions, called “ions”. These ions can combine with some antibiotics, creating an “insoluble compound,” Anastasiya Shor, an assistant professor at Touro College of Pharmacy in New York, shared via email with Live Science. Insoluble compounds do not dissolve in water, and if a drug cannot dissolve in the liquid inside the intestine, it cannot be absorbed into the bloodstream.

So, when calcium and magnesium ions are present in the intestines, they effectively trap antibiotic molecules there. The amount of antibiotics entering the bloodstream is therefore significantly reduced, reducing the effectiveness of the drug throughout the body.

This interaction with dairy products only occurs with certain antibiotics, especially those in two groups: tetracyclines and fluoroquinolones. It does not affect all antibiotics.

“To ensure antibiotics work as intended, patients should take their medication 2 hours before or 6 hours after consuming dairy products,” Shor said. “This minimizes contact between calcium and antibiotics in the gastrointestinal tract, and ensures patients receive maximum benefit from the medication.”