Barley — Food safety facts
Safe to share · Dogs · full breakdown on the linked food page
Cooked plain barley is safe and a decent fibre source.
The full verdict, portion guidance and serving notes are collected on the Barley food facts page.
Short answer
Plain cooked barley is safe for dogs and typically causes no symptoms. It's a healthy fiber source, so your dog should digest it normally. You might see a change in stool consistency due to the extra fiber, but that's not a sign of illness. Just offer fresh water and call your own vet if anything seems off.