Short answer: work with your veterinarian to run a strict elimination–challenge diet trial using either a prescription novel‑protein (limited‑ingredient) diet or a prescription hydrolyzed diet. Don’t use unprescribed blood/hair tests; the elimination trial is the only reliable way to diagnose food allergy. (vcahospitals.com)

What food-allergic dogs are commonly fed

• Novel‑protein / limited‑ingredient diets (single, uncommon protein + single carbohydrate) — e.g., rabbit + potato, venison + sweet potato, duck + peas — used when the dog hasn’t eaten that protein before. (dvm360.com)
• Hydrolyzed protein prescription diets (proteins broken into very small peptides to reduce immune recognition) — commonly prescribed brands are veterinary‑only formulas. These are an alternative when a novel protein is impractical or has failed. (meridian.allenpress.com)

Key facts your vet will use

• The only reliable way to diagnose a food allergy is a strict elimination diet followed by systematic re‑challenge of single ingredients. Other tests (blood, hair, saliva) are unreliable. (vcahospitals.com)
• Feed the elimination diet exclusively for at least 8 weeks (many clinicians recommend up to 12 weeks or longer in some cases); improvement may start earlier for GI signs but skin signs often take longer. Absolutely no other food, treats, flavored medications, chews or table scraps during the trial. (vcahospitals.com)
• Prescription veterinary diets are preferred for elimination trials; some over‑the‑counter foods have unlabeled ingredients/cross‑contamination that can invalidate the trial. (meridian.allenpress.com)
• Common canine food allergens include beef, dairy, chicken, wheat and lamb (others include soy, corn, egg, pork, fish). Once an offending ingredient is identified, strict avoidance prevents relapses. (merckvetmanual.com)
• Hydrolyzed diets help many dogs but are not universally effective; some studies show small peptides can still stimulate immune cells in a subset of dogs. If one hydrolyzed diet fails, another strategy (different hydrolyzed formula or novel protein) may be tried. (pmc.ncbi.nlm.nih.gov)

Practical step-by-step plan

1. Talk to your veterinarian (or veterinary dermatologist) — they’ll help choose a diet and rule out other causes. (meridian.allenpress.com)
2. Pick the trial diet: prescription novel‑protein limited ingredient OR a prescription hydrolyzed diet. If you prefer home‑cooking, have a veterinary nutritionist formulate a balanced recipe. (meridian.allenpress.com)
3. Transition gradually over ~7 days, then feed ONLY the trial diet (and only water) for the agreed period (usually ≥8–12 weeks). No treats, chews, flavored meds, or table scraps. Use pieces of the trial food as training treats if needed. Keep a diary of symptoms. (dvm360.com)
4. If clinical signs improve, your vet will perform controlled re‑challenges (introducing one suspected ingredient at a time) to identify the trigger(s). If no improvement, your vet may try a different elimination diet or look for non‑food causes. (vcahospitals.com)
5. Long‑term: once triggers are found, read labels carefully, avoid cross‑contaminated treats, and keep a medical alert note for pet sitters/boarding. Periodic re‑evaluation is wise because tolerances can change.

Other tips and warnings

• Don’t rely on commercial “hypoallergenic” marketing. Prescription elimination diets or veterinarian‑formulated home diets are the gold standard. (meridian.allenpress.com)
• Flavored chewable meds, supplements, and some heartworm/flea chews can contain animal proteins — ask your vet for unflavored or different formulations during the trial. (dvm360.com)
• If your dog has severe vomiting, difficulty breathing, collapse, or other signs of anaphylaxis, seek emergency veterinary care immediately.

If you want, tell me: your dog’s current food, symptoms (skin, GI, both), age/weight, and whether you prefer a commercial or home‑cooked plan — I can draft an example elimination‑diet plan and practical shopping/label‑reading tips you can show your veterinarian.