Chronic loose stool can be indicative of a variety of problems in adult dogs. Your dog could be suffering from a bacterial infection, worms or a poor diet. Changing your dog's diet is a strong first step in dealing with loose stool, though if the problem persists it may be best to speak with a veterinarian. There are several approaches to hardening up your dog's stool.
Add a bit of canned pumpkin to your dog's regular diet. One tablespoon is enough for small dogs and 2 tbsp should cover bigger breeds. This is often enough to solve the problem.
Boil a boneless chicken breast and slice it into small pieces. Mix it with some white rice and give it as a meal. The rice will calm your dog's stomach and the chicken will encourage him to eat. Once the stool gets better, switch to a high-quality dog food.
Examine your feeding habits. Dogs should eat twice a day and should not be given human food. Make sure your garbage is protected by a lid and carefully monitor how many treats you give your dog. This will keep their diet balanced and healthy.