There are a number of medications and supplements that can be helpful in reducing skin inflammation.
In mild cases, anti-histamine medication such as hydroxyzine or cetirizine is an excellent and safe choice for controlling allergies, and can be used for the long term without side-effect or danger.
In moderate to severe cases, your vet may recommend medication such as prednisolone or even depo-medrol (a long acting injection) to help relieve itching and scratching. However, these medications are corticosteroids, and are safe if used correctly, but should only be maintained for short periods as needed.
Of course before starting any medication you should discuss your pet’s condition with your veterinarian, not all medications are suitable for all pets and conditions and your vet will help you to determine the best treatment.
If a cat is so itchy that they're losing hair, have skin sores, rashes or grazes from excessive licking, then they are uncomfortable and a veterinary consultation is advisable.
However, it’s also very important to not rely overly on medication... it's much better to work on maintaining a healthy skin and coat, and reducing exposure to allergens.