If a dog who was reliably house-trained starts having accidents, begin with a veterinarian, especially when the change is sudden. Increased thirst, pain, urinary problems, aging, and other health issues can change toileting habits. It is not fair to treat a possible medical problem as disobedience.
If your dog is repeatedly straining to urinate and little or nothing comes out, seek emergency veterinary care now; do not wait for a routine appointment. Blood, pain or frequent small amounts also warrant prompt veterinary advice.
While you arrange the check
Give more frequent opportunities to go outside, note when and where the accidents occur, and clean each spot thoroughly. Keep water available. Do not punish your dog for a puddle you find later. Scolding cannot explain an accident and may make your dog hide the behavior.
After medical causes are considered
Look at changes in routine, access to the outdoors, moving house, visitors, or stress. Restart a simple schedule of supervised outings and prompt rewards outdoors. Keep notes for your veterinarian or a qualified behavior professional if the pattern continues. An older dog may need easier outdoor access and a different schedule rather than the old one repeated more firmly.
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