Quick answer: A dog drinking significantly more water than usual is a medical warning sign — the most common causes are diabetes, Cushing's disease, and kidney disease, all requiring vet evaluation.
A healthy dog drinks roughly 1 oz of water per pound of body weight per day. A 40-lb dog drinking more than 5 cups (40 oz) daily consistently, or drinking noticeably more than their normal amount, is drinking excessively. Track intake for 2-3 days by measuring what you add versus what's left.
Diabetes mellitus presents with increased thirst and urination, weight loss despite eating well, and lethargy. Cushing's disease causes increased thirst alongside a pot-bellied appearance and hair loss. Kidney disease causes increased thirst as the kidneys lose their concentrating ability. All three are diagnosed with blood and urine tests.
Don't wait for it to resolve on its own. Bring the water consumption data you tracked to the vet — it helps them assess severity before testing. Basic bloodwork and urinalysis can identify the most common causes quickly, and early treatment significantly improves prognosis.
Increased thirst in dogs is almost always medically significant. A vet visit with basic bloodwork is the right response — early diagnosis makes treatment more effective for all common causes.