Are dietary supplements helpful or harmful (Proceedings)


Although dietary supplements are thought to be safe because they are natural, this is not necessarily so.

Currently, over half of all Americans take dietary supplements on a regular basis, accounting for 15 billion dollars in sales per year!  Although the number of companion animals taking dietary supplements is not known, it appears to be growing. In magazines for pet owners and veterinarians alike, advertisements for dietary supplements abound. While it is tempting to believe the claims of disease prevention, treatment, or cure that are supposed to come from giving a few pills, knowing the real facts about supplements can help to determine which ones might be useful, which ones are useless, and which ones actually can be harmful. 

First, it is important to understand how dietary supplements are regulated.  Unlike drugs, dietary supplements do not require FDA review of product efficacy, safety, or quality prior to marketing.  The result is that the FDA has little control over supplements.  In fact, while drug manufacturers must prove the drug to be safe and effective, the FDA must prove that a supplement is unsafe.  It is easy to imagine that with the thousands of supplements on the market today, this is an unrealistic task for the FDA. Therefore, the safety, quality control, and effectiveness of dietary supplements can be questionable. 

Although dietary supplements are thought to be safe because they are “natural,” this is not necessarily so. Every year, people die as a result of dietary supplement use so safety is not a given.  What safety data is known is often known only for people.  Even if the supplement by itself is safe, its use in combination with medications an animal may be taking can cause problems by decreasing the effectiveness of the medication or by increasing its toxicity. In addition, numerous studies have shown little consistency in quality control (ie, a tablet that is supposed to contain 500 mg may contain 500 mg but also may contain more or less than that amount).  Some supplements may be contaminated (eg, fish oil supplements may contain high levels of mercury) and other issues, such as bioavailability and dissolution, can have a huge impact on the efficacy of the supplement.  As of this year, the USDA is instituting regulations that require good manufacturing practices and meet quality standards but the onus still is on the company, rather than through outside regulation. 

Another major problem is that, even if the safety and quality control issues were not an issue, there are few dietary supplements for which efficacy has been proven.  Most supplements in veterinary medicine are used based on theory, on anecdote, or on data from other species.  Properly conducted studies are few and far between for dietary supplements. Clearly, more research is needed in this area to determine which of the supplements have beneficial effects and which are a waste of money. In veterinary medicine, an additional difficulty is that the optimal (or even safe) dose is often not known and doses are empirically based on the amount given to a person.  Patients I see may be taking either extremely high doses of a supplement or may be on a dose too low to have any potential benefits. 

In people, the most commonly taken dietary supplements are multivitamins or combination multiple vitamins and minerals.  These are often taken for general health maintenance.  However, specific nutrients or botanicals often are administered in various diseases, especially cardiac, hepatic, renal, and orthopedic disease, as well as cancer. 

Because the use of dietary supplements is becoming more common, it is important to specifically ask owners if they are giving them to their pets.  Owners often do not consider dietary supplements to be either a part of the diet or a medication so may not offer the information unless specifically asked.  Once you find out what owners are giving their pets, you may then need to gather additional information.  For general information on dietary supplements and to determine which supplements are safe, what doses might be appropriate, and which have been tested for quality control, you might find the following websites/resources useful.


Websites (test dietary supplements for purity, potency, bioavailability, etc)

FDA (regulatory and safety issues, adverse event reporting, meetings, industry information)

Mayo Clinic Drugs and Supplements Information

NIH National Center for Complementary and Alternative Medicine

NIH Office of Dietary Supplements (fact sheets, safety notices, database)

Quackwatch ("Guide to health fraud, quackery, and intelligent decisions")

USDA Food and Nutrition Information Center (general supplement and nutrition information, links to a variety of dietary supplement websites)

United State Pharmacopeia Dietary Supplement Verification Program<


AAFCO Official Publication (

Bragg RR, Freeman LM, Fascetti AJ, Yu Z.  Composition, disintegrative properties, and labeling compliance of commercial taurine and carnitine supplements.  J Am Vet Med Assoc 2009; 234: 209-213

Freeman LM, Abood SK, Fascetti AJ, et al.  Disease prevalence among dogs and cats in the United States and Australia and proportions of dogs and cats that receive therapeutic diet or dietary supplements.  J Am Vet Med Assoc 2006;229: 531-534

Lana SE, Kogan LR, Crump KA, et al.  The use of complementary and alternative therapies in dogs and cats with cancer.  J Am Anim Hosp Assoc 2006; 42: 361-365.

National Research Council. Nutrient requirements of dogs and cats, 2006.  National Academies Press.

PDR for Nonprescription Drugs, Dietary Supplements, and Herbs, 2010. Thomson Healthcare Publishers.

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