Magnesium compounds Quinolones

The serum levels of many of the quinolone antibacterials can be reduced by aluminium and magnesium antacids. Calcium compounds interact to a lesser extent, and bismuth compounds onfy minimal. Separating administration by 2 to 6 hours where significant interactions occur reduces admixture in the gut and can minimise the effects. 

It is believed that eertain of the quinolone functional groups (3-carboxyl and 4-oxo) form insoluble chelates with aluminium and magnesium ions within the gut, which reduces their absorption. " The stability of the chelate formed seems to be an important factor in determining the degree of interaction. It has been suggested from animal studies that adsorption of quinolones by aluminium hydroxide re-precipitated in the small intestine may be a factor in the reduced bioavailability of quinolones. See also Quinolones + Iron or Zinc compounds , p.336. 

Didanosine is extremely acid labile at pH values below 3, so one ofthe formulations contains buffering agents (dihydroxyaluminium sodium carbonate and magnesium hydroxide) to keep the pH as high as possible to minimise the acid-induced hydrolysis. Ciprofloxacin forms insoluble non-absorbable chelates with these metallic ions in the buffer so that its bioavailability is markedly reduced. See also Quinolones + Antacids or Calcium compounds , p.328. 

Food, dairy products and calcium compounds markedly reduce the absorption of strontium ranelate, and administration should be separated by at least 2 hours. Aluminium and magnesium antacids only slightly reduce strontium ranelate absorption. Strontium ranelate is predicted to reduce the absorption of the quinolones and the tetracyclines, and strontium should be stopped during courses of these antibacterials. Vitamin D does not affect strontium ranelate bioavailability. 

---