I was just reading a review of multivitamin/multimineral supplements by a scientist with the Office of Dietary Supplements at the National Institutes of Health. Elizabeth Yetley (2007) attempted to determine what we could garner from the large reviews about supplement use in people, focusing on just these multis, rather than other nutraceuticals or botanicals. The paper essentially focuses on the uncertainties regarding how the actual dose varies from label claim (let's just say that's a wide variation above and below what the label says). It also touches on what we don't know about bioavailability (absorption, utilization) and bioequivalence (differences in activity between different chemical forms) between the various forms of vitamins and minerals.
Bioavailability is affected by the person (or animal) taking the supplement, and not just the product itself. Every individual has genetically determined differences in their ability to absorb and metabolize nutrients, which clearly suggests that individuals have DIFFERENT nutritional requirement for the same nutrient. Further confounding these differences are the effects of various life stages and conditions - sex, age, and medical condition all change the way nutrients are handled metabolically. And the drugs being taken for those medical conditions impose even more obstacles to what would otherwise be considered 'normal metabolism' of any single nutrient.
A large host of drugs can affect absorption and utilization of nutrients, and vice versa. Just SOME of these interactions of importance to veterinary medicine are [HINT - if you are administering any vitamin, mineral, or drug to your pet, you can visually scan the lists below, or just search for the generic or chemical name on this web page. Press Ctrl F and type the generic or chemical name into the search box for either Explorer or Firefox]:
Pancreatin and sulfasalazine - decrease absorption of folate
Omeprazole (Prilosec) - reduce absorption of B12, and reduces bioavailability of iron
Aluminum hydroxide - along with extra Vitamin C, can increase the potential for aluminum toxicity. It can also decrease absorption of iron.
Vitamin C - reduces the amount of B12 available for serum and body stores
Pau d'arco - may reduce effectiveness of vitamin K
Calcitriol - can increase absorption of magnesium and calcium to dangerous levels
Digoxin - can decrease blood levels of magnesium
Doxycycline - can decrease the effectiveness of iron supplementation
Calcium absorption may be reduced by: atenolol, cefpodoxime (Simplicef), ciprofloxacin, enrofloxacin (Baytril), doxycycline, ketaconazole, itraconazole, sucralfate, tetracycline.
Magnesium supplementation may reduce the effectiveness of: allopurinol, azithromycin, cefpodoxime (Simplicef), cimetidine (Tagamet), ciprofloxacin, doxycycline, gabapentin, iron, itraconazole, thyroid hormone (thyroxine), misoprostol, penicillamine, sotalol, sucralfate, tetracycline
Zinc - decreases iron and copper absorption, and decreases the effectiveness of ciprofloxacin
Iron supplementation can decrease the effectiveness of: ciprofloxacin, doxycycline, penicillamine.
Complete and balanced pet foods do contain supplemented vitamins and minerals, and these levels are probably not causing drug interactions. The question to be asked, and that we have no answer for that I know of, is whether therapy using these drugs interacts with these 'adequate' nutrient levels, rendering them inadequate. We do know that some sick animals have lower levels of certain nutrients in their blood than healthy ones – for instance, cats with kidney disease have lower levels of Vitamin E and cats with diabetes lower levels of magnesium. Another reason why nutrition, and often a veterinary nutritionist, should be involved when your pet has a chronic illness.
Yetley EA. Multivitamin and multimineral dietary supplements: definitions, characterization, bioavailability, and drug interactions. Am J Clin Nutr. 2007 Jan;85(1):269S-276S.