Unlike medications, supplements are not regulated by the FDA. While this regulatory agency frowns upon supplement manufacturers making unproven claims about their products, some companies may skirt this line to draw you in. Talk with your veterinarian before beginning a supplement. He or she is your best resource regarding the safety and effectiveness of vitamins, nutraceuticals, herbs, or other additives.
When considering a purchase, read the label. All labels should have lot numbers, the name of the manufacturer and a website or phone number for contacting them.
Look for a complete list of active and inactive ingredients (including amounts of each) on the label.
Read the website and look for information about possible drug interactions and side effects.
Look for a seal from the National Animal Supplement Council. The NASC has standards for labeling, quality assurance, and how to report any side effects from the use of supplements. NASC member manufacturers are subject to audits and quality control testing.