There are a few important things to remember if you are going to try a hypoallergenic diet trial for your pet's allergies:
It can take 6-8 weeks to note any improvement in skin/itching once the diet has been changed.
If there is a current bacterial or yeast/fungal skin infection, it will not go away simply by changing the diet! Medical treatment is needed to eliminate the infection BEFORE we can know if the diet will help prevent future infections.
Nothing else should be given to the dog by mouth except the diet initially. This includes rawhides, table food, and treats. The only exceptions can include baby carrots, green beans, and sweet potatoes (Sam's Yams makes a dehydrated sweet potato treat in a variety of sizes...or you can bake your own!). Ideally, even flavored medications such as heartworm chewable tablets, should be changed to hypoallergenic options (Revolution and Advantage Multi are topical products than can be used to treat heartworms; also, Heartgard does make an unflavored tablet that usually needs to be ordered by your vet).
Remember that four weeks of a well-followed diet trial followed by one accidental chicken nugget starts you back at day zero of the diet trial...
There are three main types of hypoallergenic diet options:
Hydrolyzed Protein Diet - the proteins in this food are broken down into tiny, tiny pieces that are too small for the immune system to recognize as something to cause an allergy. These diets are not available over the counter; they are: Royal Canin Hypoallergenic HP (hydrolyzed protein) Adult Diet, Hills Z/D Ultra
Novel Protein Diet - this approach works by feeding a protein source to which the pet has not previously been exposed (and therefore has not formed an allergy to). Examples of prescription novel protein diets are: Royal Canin Hypoallergenic Selected Protein PV Diet
Vegetarian Diet - the most common food allergens for dogs are meats (chicken, beef). Vegetarian diets use soy as a protein source. Examples include: Royal Canin Vegetarian, Purina HA
Why not use an over the counter option for my dog?
Over the counter diets are not processed with the same rigorous standards as veterinary prescription diet (e.g. cleaning of machines between preparation/packaging of one type of diet and another). They may be successful for some dogs, but a failure to do well on an over the counter diet does NOT mean that a diet trial "won't work". It may simply mean that a veterinary prescription diet is necessary. I recommend starting with a prescription diet to get symptoms under control and to find out if this process will be successful for your pet, and then considering changing to an over the counter diet at a later point if this is something that interests you. If signs worsen again, then you will be able to tell that the over the counter diet is not an option. When choosing an over the counter diet, it is important to make sure that you read ingredients carefully and don't rely on package advertising. Many diets will advertise being "limited ingredient" but are then preserved with poultry fat! This introduces chicken, and defeats the purpose of the limited ingredient diet. Look for a diet preserved with Vitamin E instead of poultry fat.