Dangerous Ingredients in Supplements

Laurie Anne Walden, DVM

Photo by Leohoho on Unsplash

Dietary supplements can contain ingredients that aren’t safe for dogs and cats. These products are available without a prescription and often marketed as “natural,” so you might assume they’re not dangerous. But dietary supplements are not FDA regulated for safety and effectiveness, and their formulations can change without notice.
 
The following is a short list of potentially dangerous ingredients in supplements sold for animals and for humans. With supplements intended for animals, the main risk is overdose. If you have one of these products, be sure your pets don’t have access to the whole container (especially with flavored products). With supplements intended for humans, even a small dose of a problem ingredient can pose a serious risk to animals.
 
Supplements for Animals
L-Tryptophan
Some products marketed as calming supplements for pets contain L-tryptophan. Large amounts of L-tryptophan can cause trouble with the digestive and nervous systems. Signs in dogs include vomiting, excessive drooling, tremors, and coordination problems.
 
Methionine in “Grass Saver” Supplements
Supplements to reduce grass discoloration caused by urine are available even though they aren’t likely to work. These supplements contain methionine, which makes urine acidic. High doses of methionine cause tremors and seizures.
 
Glucosamine and Manganese
Pet joint supplements that contain glucosamine and chondroitin are generally safe. However, these supplements can also contain manganese, and an overdose of manganese can cause liver failure. Many joint supplements are flavored chews, increasing the risk of overdose. An overdose of flavored chews can also lead to bloating and shifts in blood electrolyte levels.
 
Supplements for Humans
Alpha-Lipoic Acid
Alpha-lipoic acid (ALA) is an antioxidant included in supplements to support a variety of conditions in people. ALA might also be in joint supplements for dogs. Cats are much more susceptible than dogs to the toxic effects. Even small amounts of ALA are dangerous to cats. High doses of ALA in dogs and cats can cause vomiting, low blood sugar, liver problems, tremors, and seizures. Overdose can be fatal.
 
Iron
Supplements for humans are the main source of iron exposure for pets. Iron levels are higher in prenatal vitamins and iron supplements than in typical multivitamins. In dogs and cats, iron toxicosis causes stomach pain, vomiting, diarrhea, and liver failure.
 
Vitamin D3 (Cholecalciferol)
Although vitamin D is necessary for healthy bones and muscles, it’s toxic in high doses. Human vitamins are one source of exposure for pets; cholecalciferol is also in some mouse and rat poisons. Cholecalciferol toxicosis causes vomiting, diarrhea, and kidney damage.
 
Caffeine
Caffeine is often in supplements marketed for energy or weight loss. Products containing green tea extract, coffee bean extract, or guarana contain caffeine. In dogs and cats, caffeine can cause vomiting, restlessness, increased heart rate and blood pressure, tremors, and seizures. Very high doses can be fatal.
 
Xylitol
Xylitol is a natural sweetener in many flavored products, including flavored chewable supplements for people. Xylitol isn’t harmful to people or cats. In dogs, though, xylitol causes a dangerous drop in blood sugar and can also damage the liver. If untreated, xylitol toxicosis can be fatal to dogs.
 
 
Sources

• Larkin M. Supplements’ toxicity risks not easy to determine as new, reformulated products hit shelves. AVMA. October 8, 2025. Accessed December 9, 2025. https://www.avma.org/news/supplements-toxicity-risks-not-easy-determine-new-reformulated-products-hit-shelves
• Magnuson M. Too much of a good thing: when pet supplements turn toxic. The Conscious Vet. November 7, 2025. Accessed December 9, 2025. https://theconsciousvet.substack.com/p/too-much-of-a-good-thing-when-pet
• Which supplements and vitamins are dangerous for pets? ASPCA. January 15, 2021. Accessed December 9, 2025. https://www.aspca.org/news/which-supplements-and-vitamins-are-dangerous-pets

 
Image source: https://unsplash.com/photos/brown-and-yellow-medication-tablets-DRchVK5apjw

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