Keratin is a structural protein your body makes in hair, skin, and nails. Supplements are usually hydrolyzed from animal sources. Your digestive system treats it like any other protein.
People take keratin capsules or powders for brittle nails or thinning hair. You can take it with or without food. Because digestion breaks it down, the protein's direct effect on hair and nails is not backed by strong evidence.
Keratin supplements are safe for most adults. A few people notice mild stomach upset. No drug interactions are documented. If you have an allergy to animal products, check the supplement's source.