Discussing putting animals to sleep can be emotional. Use the phrase compassionately around pet owners. When her cat was in pain, the vet gently explained why they should put it to sleep.
Pro Tip 2/3
Not Reversible
'Put to sleep' in this context means it's a permanent decision for the animal, unlike for humans. After many treatments failed, it was humane to put the horse to sleep.
Pro Tip 3/3
Serious Illness
Used when an animal cannot recover or its quality of life will be too poor. The dog couldn't walk anymore, so they had to put it to sleep.