Short answer: for chronic diarrhea in dogs, veterinary evaluation plus a strict elimination trial with either a hydrolyzed protein diet (first-line for many cases) or a limited-ingredient novel-protein diet is the usual approach. Hydrolyzed/elemental diets have good evidence for food‑responsive chronic enteropathy; if you try an LID, feed it exclusively for 8–12 weeks and remove all treats/toppers/chews. See below for practical options, how to run the trial, and when to see a vet. (onlinelibrary.wiley.com)

1. When to see a vet (do this first)

• If the dog has systemic signs (fever, severe lethargy, weight loss, repeated vomiting, blood in stool), get urgent veterinary care.
• For chronic or recurrent diarrhea (weeks to months) a basic diagnostic workup is recommended before or while starting diet therapy: fecal tests (parasites, Giardia), CBC/chemistry, serum cobalamin (B12) and folate, pancreatic tests (TLI/cPLI) and abdominal imaging as indicated. These tests both rule out other causes and help predict prognosis (e.g., low cobalamin can occur with chronic enteropathies). More invasive testing (endoscopic/surgical biopsies) is reserved for dogs that don’t respond to diet/medical trials. (pubmed.ncbi.nlm.nih.gov)

2. Which “limited ingredient” approach to choose

• Hydrolyzed protein diets: many specialists recommend trying a veterinary hydrolyzed diet first for dogs with chronic enteropathy because the proteins are broken into very small peptides that are less likely to trigger an adverse reaction. Multiple clinical trials show hydrolyzed diets can produce remission of signs in a large proportion of dogs. Examples (veterinary‑only) include Hill’s z/d, Royal Canin hydrolyzed formulas, and Purina HA. Use these under your veterinarian’s guidance. (onlinelibrary.wiley.com)
• Limited‑ingredient novel‑protein diets (single protein + single carbohydrate): an alternative if the dog won’t eat the hydrolyzed food or if hydrolyzed diet fails. These include both prescription and good-quality OTC LID lines. If you use an OTC LID, be aware of small‑amount cross‑contamination risks in manufacturing—prescription therapeutic diets are made under stricter controls. (purinainstitute.com)
• Elemental (amino‑acid) diets are used when dogs fail hydrolyzed or novel diets; they can be effective but are typically specialty/clinic diets. (link.springer.com)

3. How to run an elimination (limited‑ingredient) trial correctly

• Duration: feed the chosen diet exclusively for at least 8 weeks and ideally up to 12 weeks (improvement may be seen within 1–4 weeks for GI signs but allow the full trial to be confident). Don’t give any other food, table scraps, flavored chew toys, or treats — even small amounts can invalidate the trial. If the dog improves, a challenge with the old diet can confirm food‑responsiveness. (academy.royalcanin.com)
• Transition: switch gradually over ~7 days (start with ~25% new food, increase daily) unless your vet advises a faster switch because of severity. Keep water available; measure portions. (hillspet.com)
• Strictness: medications or flavored supplements may contain protein flavors—work with your vet on alternatives (e.g., unflavored medications, pill pockets approved for the diet, or giving meds in food only if the food is the elimination diet). (academy.royalcanin.com)

4. Practical diet choices (examples available in the U.S.)

• Hydrolyzed prescription diets (common examples): Hill’s Prescription Diet z/d (hydrolyzed), Royal Canin Hydrolyzed Protein veterinary formulas, Purina Pro Plan Veterinary Diets HA (hydrolyzed). These are intended for elimination trials and management of food‑responsive enteropathy. Choose the specific formula your veterinarian recommends based on whether you need low‑fat, low‑phosphorus, etc. (hillspet.com)
• Over‑the‑counter limited‑ingredient diets: several commercial LID/kibble and canned lines exist (single protein/carb formulas). If you use OTC LID, pick one the dog has never eaten, and be aware of cross‑contamination/manufacturing limitations—prescription diets are preferred for strict diagnostic trials. (purinainstitute.com)

5. Adjuncts that may help

• Probiotics: some probiotics can help normalize stool for acute or intermittent diarrhea and are commonly used as adjunctive support in chronic cases; evidence is mixed but some products (e.g., Enterococcus faecium preparations like FortiFlora) are widely used in veterinary practice. Discuss species‑appropriate probiotics with your vet. (pubmed.ncbi.nlm.nih.gov)
• Fiber: if signs point to large‑bowel diarrhea (mucoid stools, straining, urgency), soluble fiber (psyllium, canned pumpkin) or a fiber‑enhanced diet may help. Don’t add fiber blindly—ask your vet which type/amount is right. (veterinary-practice.com)

6. When diet doesn’t fix it

• If a strict elimination trial plus appropriate tests and supportive care don’t produce adequate improvement, your vet may try an antibiotic trial (e.g., tylosin for some dogs), immunosuppressive therapy, or recommend GI biopsies to determine inflammatory bowel disease versus other pathology. Persistent or severe cases (weight loss, hypoalbuminemia, severe hypocobalaminemia) need more aggressive diagnostics and specialist input. (pubmed.ncbi.nlm.nih.gov)

7. Practical checklist you can follow now

• Book a veterinary visit for baseline diagnostics (fecal, bloodwork, cobalamin/TLI as indicated). (pubmed.ncbi.nlm.nih.gov)
• Pick a single veterinary hydrolyzed prescription diet (e.g., Hill’s z/d, Royal Canin hydrolyzed, Purina HA) or a true novel‑protein LID your dog has never eaten. Feed nothing else for 8–12 weeks. (hillspet.com)
• Use an evidence‑based probiotic/topper only if recommended by your vet (e.g., FortiFlora is a common veterinary option). (purina.com)
• Keep a stool log (consistency, frequency, appetite, weight) and report progress to your vet at 2–4 week