Oral medications like ciprofloxacin need to make their way past the gastrointestinal barrier and into the body to be effective. Some foods and drinks, if ingested at the same time as the medication, can interfere with the absorption of certain drugs. Ciprofloxacin with milk is one of these situations, and therefore doctors do not advise this combination.
Every drug has what is called "bioavailability." This is the percentage of the drug in the delivery method that manages to make its way into the blood circulation of the body. The percentage of the drug that is not included in the bioavailability may be broken down into other molecules, or may flush out of the body through urine or feces.
A 1991 study by the University of Turku in Finland analyzed the bioavailability of ciprofloxacin with milk, and other dairy products. Each participant in the study fasted overnight, and then ate yogurt, or drank milk or water along with a dose of ciprofloxacin. The researchers tested the plasma portion of the participants' blood over time. They found that those patients who only drank water had a higher concentration of drug circulation than those who ingested ciprofloxacin with milk or ate yogurt. The drug plasma concentrations of the milk drinkers were nearly half that of the water drinkers.
In 2010, researchers at Semmelweis University in Hungary found that the most likely culprit for this lack of absorption through the gastrointestinal tract was a protein called casein. Casein is a major component of milk and milk products like yogurt, and when it comes into contact with the drug it hampers absorption into the body. Manufacturers of ciprofloxacin products take this research into account when they write the product instructions for users.
All drugs, including antibiotics like ciprofloxacin, need to be present in the body at specific concentrations in order to do their job properly. Ciprofloxacin kills certain types of bacteria, such as those that cause legionellosis and some food poisonings, and low concentrations may not clear the infection. Doctors, therefore, need to know how much of a drug will do the job properly, and prescribe the dose accordingly.
Ciprofloxacin with milk together requires a patient to take more of the drug to cure an infection. As nearly all drugs have side effects, doctors do not wish to prescribe more medication than necessary. The risk of side effects from this particular drug include such dangerous complications as hallucinations, ruptured tendons or alterations in heartbeat. Possible complications for a patient who takes ciprofloxacin with milk can include a worsening of the infection.