One of the most common questions I get at my clinic runs along the following lines....
"Hey, I just noticed my dog is constipated. What can I give him for that?"
I'm sure that there will be some people that won't really read through the next parts, so if you found this blog entry by doing an internet search for the problem, please go through this carefully (and read my disclaimer sidebar).
In most cases where an owner thinks their dog or cat is constipated, they actually aren't. In fact, they probably have diarrhea. I don't have data on this, just nearly 17 years of being a practicing vet and numerous cases. In almost every case of "straining to have a bowel movement" assumed to be constipation it turns out that the stool is actually very loose. And giving medicine for constipation will make the problem worse.
When the colon is irritated or inflamed the body responds by increasing the strength and frequency of contractions. The body is trying to expel whatever caused the problem. Unfortunately the contractions will continue even if there are no feces to pass. Stop and think about this for a moment because we've all been there. All of us have had times when we've had a virus, food poisoning, or just something that really upsets our bowels. We're sitting there on the toilet having rather uncomfortable straining and contractions even though nothing is coming our or getting stuck. If you took a laxative at that moment, how do you think you're going to feel?
Don't get me wrong....animals can get constipated. Cats are especially prone to a condition called megacolon where the lower GI tract becomes distended and cannot push feces out, resulting in dehydration of the stool and constipation. If you ignore constipation you could be leading to more serious problems. However, if you treat diarrhea as if it were constipation you're going to also cause worse problems.
So what do you do? How do you tell if the pet is truly constipated? Well, you have two options. 1) Put your finger in your dog's/cat's rectum, palpate the abdomen, and see if there is blocked or hard feces. 2) Take your pet to the vet. I certainly don't recommend option #1 as you could potentially hurt your pet, they won't like it, and most people don't know what "normal" is like in order to make the proper assessment. That only leaves #2. Yes, I know that's not what you wanted to hear, otherwise you wouldn't be looking for advice from "Dr. Google". However, that's absolutely the best option.
The take-home lesson here is that what looks like constipation isn't always constipation, and you need a veterinary exam in order to make the best assessment.