Loose stools can occur for a number of reasons. It is best, when possible, to determine the cause of the loose stool and then treat the cause.
In almost can case of soft stools or diarrhea it is a good idea to have the stool examined to make sure that there are no signs of intestinal parasites and to consider checking fecal smears for signs of bacteria associated with intestinal problems.
If these tests are negative it is sometimes worthwhile to try one of the safer de-wormers, such as fenbendazole (Panacur Rx) anyway, just to be cautious. A blood chemistry panel and complete blood cell count are also reasonable tests, to be sure that there is not a contributing systemic cause such as kidney or liver problems.
There are several things to think about while looking for the cause of the diarrhea. The first thing to think about is whether your dog is maintaining weight. If that's the case, there is less need to rush into sophisticated testing procedures if nothing shows up on the initial testing that your vet can do.
The next thing to think about is the pattern of the diarrhea or soft stools. If the problem is originating in the small intestine, the frequency of bowel movements tends to be normal, or about 2 - 4 bowel movements per day for a puppy, 2 for an adult. If there are frequent bowel movements and soft stools or diarrhea then the problem is more likely to be in the large intestine. Knowing the pattern of the bowel movements can help your vet in deciding what tests to pursue.
If your vet has done the routine testing and you wish to try dietary means for controlling the soft stools we have the best luck using Hill's w/d (tm) diet. This is a low fat, moderate fiber diet. You might be able to use Fit and Trim (tm) or Cycle Lite (tm) or some similar food, as well. An alternative is to use any low fat diet and add fiber (several companies make low fat diets). Fiber can be added by using high fiber cereals, canned pumpkin or psyllium (Metamucil and other products). Puppies have done well when fed these diets for several weeks but it is best to see if you can switch to a puppy food after the stools are normal for a week or so. When these diets help they usually help pretty quickly, within a few days. If they don't help it would be worth trying the hypoallergenic diets (Purina LA tm or Hill's z/d tm) but these have to be used for at least 3 – 4 weeks before giving up on them, in order to really rule out the possibility of a food allergy.
If you get through the routine testing and easy dietary trials without a cure, it would be a good idea to do some more sophisticated testing. At least consider a trypsin-like immunoreactivity (TLI) test to be sure that there isn't a problem with deficient digestive enzymes. A bile acid test to rule out liver disease might be a good idea in this circumstance, too. Endoscopic examination of the digestive
tract, with biopsy samples taken during the procedure, is another helpful test procedure.
Hopefully things are already better and you don't need this advice.
Advanced testing for puppies with soft stools who are growing normally and doing well otherwise and who show no signs of discomfort from the soft stool production is probably not required. I think it is probably best but I have a number of patients with consistently soft stools who seem to be doing fine otherwise, so I tend to think it is possible for dogs to live a normal lifestyle despite having this problem.