Ciprofloxacin is an antibiotic prescribed to treat various infections, including urinary tract infections and some cases of acute sinusitis (inflammation of the sinuses). The drug is sold under a number of different brand names but is most widely known by the name Cipro. Regardless of the brand name, ciprofloxacin should never be taken in combination with milk or other dairy products, which can interact with the drug to make it less effective.

Is This an Emergency?

If you are experiencing serious medical symptoms, seek emergency treatment immediately.

What Is Ciprofloxacin?

According to Bayer's prescription information, ciprofloxacin is a broad-spectrum antibiotic (meaning it can kill or inhibit a wide range of different bacteria). It's one of a class of drugs called quinolones that attack bacteria by binding to enzymes called topoisomerases. Topoisomerases wind and unwind DNA during replication(when a cell is copying its DNA prior to division). By binding to bacterial topoisomerases the ciprofloxacin halts DNA replication and kills the bacterium.

Bioavailability of Ciprofloxacin

Bioavailability is a measure of the percentage of a drug administered to a patient that actually reaches general circulation in the bloodstream. If a drug is administered via IV, it has a bioavailability of 100 percent. If it's administered orally or via another route, however, some of the drug molecules may not be absorbed through the intestinal lining, while other drug molecules may be metabolized and broken down by the liver, so only a fraction of the drug molecules will reach systemic circulation. That fraction is defined as the bioavailability. According to the manufacturer, Bayer AG, Cipro's absolute bioavailability is 70 percent.

Ciprofloxacin and Milk

Bayer cautions patients that ciprofloxacin shouldn't be taken together with milk. In a study published in the journal Clinical Pharmacological Therapy in 1991, researchers gave patients ciprofloxacin together with milk or yogurt to see how the combination altered the drug's effectiveness. The researchers concluded dairy products reduce the bioavailability of ciprofloxacin (the fraction that reaches general circulation) by 30 to 36 percent and the peak concentration by 36 percent. Drinking milk or eating dairy products while you're taking ciprofloxacin dramatically reduces the amount of the drug that will actually reach the bloodstream--the place where it needs to be.

Resistance to Ciprofloxacin

Just as with other antibiotics like penicillin, bacteria can develop resistance to ciprofloxacin. Treating an infection with low or insufficient concentrations of an antibiotic can increase the chance the bacteria will develop resistance; if the concentration isn't high enough to prevent mutants that are less susceptible to the antibiotic from surviving and dividing, these mutants will pass on the genes that make them less susceptible and the population of bacteria could become more resistant over time. Taking ciprofloxacin together with milk or dairy products reduces the peak concentration of the drug in the bloodstream and thus increases the chances that the bacteria might become resistant.

Why Does Milk Reduce the Bioavailability of Ciprofloxacin?

It's believed that the calcium in the milk interacts with the ciprofloxacin molecule and binds to it in a way that prevents it from being absorbed; consequently, it's also recommended that you refrain from drinking calcium-fortified orange juice while you're taking ciprofloxacin as well. According to a study published in the Journal of Pharmaceutical and Biomedical Analysis in 2010, casein (a protein found in milk) may also bind with ciprofloxacin and make it less effective 1.