Common pet supplements include joint support (glucosamine, chondroitin), omega-3 fish oil, probiotics, calcium powder for reptiles, and vitamin C for guinea pigs (who cannot synthesize their own). Supplements come as powders, chews, liquids, or capsules. Over-supplementation can be as harmful as deficiency. Excess calcium in large-breed puppies can cause skeletal problems, and too much vitamin A is toxic to cats. Pets eating a complete-and-balanced commercial diet rarely need additional supplements unless a veterinarian identifies a specific need. Always consult your vet before adding supplements, especially alongside prescription medications, as interactions can occur.