Why does milk reduce the effectiveness of antibiotics?

Research by scientists shows that some antibiotics when taken with milk can reduce their effectiveness, especially those treating bacterial infections.

Oral antibiotics - in pill or liquid form - 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 (brand name Declomycin) into the blood 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.

Research shows that drinking milk before taking antibiotics can reduce the effectiveness of the drug. (Photo: UniversalImagesGroup/Getty).

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 drug.

Another study, published in the journal Acta Pharmacologica et Toxicologica , found that the drug sotalol (brand name Betapace) , used to treat irregular heartbeats, was also combined with calcium. This combination reduced the absorption and effectiveness of the drug when taken with milk.

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

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

Scientists think milk interferes with the absorption of these drugs because it contains calcium and magnesium ions, known as "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, told Live Science by email . Insoluble compounds don't dissolve in water, and if a drug can't dissolve in the fluid inside the intestine, it can't be absorbed into the bloodstream.

So when calcium and magnesium ions are present in the intestine, they effectively trap the antibiotic molecules there. The amount of antibiotic entering the blood 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 the antibiotic works as intended, patients should take the medication 2 hours before or 6 hours after consuming dairy products ,' says Shor. 'This minimizes contact between calcium and the antibiotic in the gastrointestinal tract, and ensures the patient gets the maximum benefit from the medication . '