<p>Concomitant intake of tetracyclines and zinc may decrease the Gl absorption and serum levels of tetracyclines. Similarly concomitant administration of zinc and fluroquinolones may decrease the Gl absorption and serum&nbsp; levels of some fluroquinolones. Coadministration of Niacin and HMG-CoA reductase inhibitors (eg. lovastatin) may result mayopathy and rhabdomyolysis. Pyridoxine reduces levodopa&#39;s effectiveness by increasing its peripheral metabolism. Co-administration of pyridoxine with phenytoin may decrease serum levels of phenytoin.</p>