Inability to urinate is a medical emergency. Your dog needs to be examined by a veterinarian. The inability to urinate is more common in cats that dogs but can occur in dogs. Causes can include bladder stones that have become lodged in the urethra, masses (prostatic or bladder) that cause constriction, neurological problems, or trauma to the penis or prepuce. After examination your dog may need x-rays or an ultrasound and will have his bladder voided via catheter or cystocentesis.
