VetInfo points out that the U.S. Food and Drug Administration has not approved the use of hydrocortisone cream for dogs, and it is not always a safe treatment. Dogs should only be treated with hydrocortisone cream under the advice and supervision of a veterinarian.
Mercy Animal Hospital explains that while steroid treatments such as hydrocortisone cream are highly effective at treating itching, they are not safe for long-term use. A dog that experiences seasonal itching for as little as three weeks may be treated with hydrocortisone cream for that short period of time, but the side effects of steroids mean that longer treatment is ill-advised. About.com points out that dogs may end up licking areas treated with hydrocortisone cream, which may be toxic.