Drug Interactions Newsletter

Volume 1, Issue 18, 05/19/2009

Question

How do calcium supplements (such as calcium carbonate) significantly reduce the absorption or bioavailability of ciprofloxacin (Cipro®) thereby increasing the risk of therapeutic failure?

Answer

Daily use of calcium containing supplements (such as calcium carbonate) or medications are fairly common and are known to be recommended by some medical authorities.(1)  Some patients may fail to disclose their use during a review of medications at a clinic visit or upon admission to the hospital because they may not consider the supplement to be a medication.  Daily administration of calcium carbonate is usually safe and appropriate, but problems can arise when the patient develops an infection that requires the use of the antibiotic, ciprofloxacin (Cipro®).  This is a very plausible situation in both the outpatient setting and hospital setting.  For patients in the outpatient setting, it is common and maybe even desirable for the patient to take all of their medications at one time.  In the hospital, administration of multiple medications simultaneously may also desirable.  Conversely, it may be that the scheduling of medication administration is simply not given enough attention in regard to avoiding combinations of drugs that should not be taken at the same time.  Regardless of the reason, the coadministration of calcium carbonate and ciprofloxacin can result in a reduction in the absorption of ciprofloxacin by an average of 40%.(2)  Absorption can be further reduced if the gastric pH is increased by other medications such as H2 receptor antagonists or proton pump inhibitors.(3)  What does calcium do to ciprofloxacin that results in this reduction in absorption or bioavailability?

A basic evaluation of the chemical structure of ciprofloxacin reveals that each ciprofloxacin molecule has......To read the full answer please LOGIN or SUBSCRIBE NOW.

Other keywords found in this issue:  gastric pH, pKa, ionized, chelation, cation, MIC, infection, mechanism