Why has my cat suddenly stopped using the litter box?
Quick answer
A sudden refusal to use the litter box is most often a sign of an underlying medical problem, so the first step is a vet visit rather than assuming it is purely behavioural.
The full answer
Pain teaches a cat to associate the tray with discomfort, and that association can outlast the illness itself. That is why treating the medical cause and rebuilding the habit usually both need to happen.
Rule out medical causes first
Conditions that commonly trigger litter box avoidance include urinary tract infections, bladder stones or crystals, feline lower urinary tract disease (FLUTD), kidney disease, diabetes, constipation and diarrhoea.
Arthritis counts too: a cat that finds it painful to climb into a tray with high sides may simply stop using it. A vet visit with a urine sample, and often blood work, is the fastest way to sort medical causes from behavioural ones.
The emergency you cannot wait on
Male cats deserve urgent attention. A urethral blockage is life-threatening, and a cat can deteriorate within a day. Signs include straining in the tray with little or no urine produced, crying out, frequent visits, licking the genital area, vomiting or hiding.
If you see any of these, contact an emergency veterinary service immediately rather than waiting for morning. Blood in the urine, or a cat that has not urinated for many hours, also warrants urgent care.
Behavioural and environmental triggers
If your vet confirms the cat is healthy, look for what changed. Common triggers are a dirty tray, a change of litter type or brand, a covered box, the box being moved, a new pet or a new baby, a house move, building work, or a frightening event while the cat was using the tray.
Household tension matters too: a new cat in the neighbourhood visible through a window, or conflict between cats in the home, can make a tray in a particular spot feel unsafe. Note when the behaviour started and what else changed around that date.
Fixing the setup
The usual guideline is one tray per cat plus one extra, placed in quiet, separate, easily accessible locations rather than lined up together. A tray about one and a half times the cat's body length, with low sides and no lid, suits most cats.
Use an unscented fine-grained clumping litter, keep it to a comfortable depth, and scoop at least once a day with a fuller change of litter regularly. Avoid liners and deodorisers, which many cats dislike, and make sure an older or arthritic cat can step in without jumping.
Cleaning accidents and retraining
Clean soiled spots with an enzymatic cleaner and avoid ammonia-based products, because ammonia smells like urine and encourages the cat to return to the same place. Do not use steam cleaners on urine, which can set the smell in.
Then go back to basics: confine the cat to a small room with a clean tray for a few days, feed on a schedule, place the cat gently in the tray after meals and waking, and reward any use calmly. Never punish the cat or rub its nose in the mess, which reliably makes avoidance worse.
Lowering stress in a multi-cat home
In households with several cats, resources are often the real issue. Separate feeding stations, several water bowls, multiple scratching posts and trays in different rooms reduce competition and give each cat a route that does not pass another cat's territory.
A pheromone diffuser can help lower tension, and adding vertical space such as cat trees and shelves makes it easier for cats to avoid each other. If tension persists, ask your vet or a feline behaviourist for help rather than waiting for it to settle.