possible digestive problem

Question
My 10 year old, 20 pound neutered male shih tzu has only had one very small bowel movement in the last 36 hours.  He is urinating as usual, and behavior seems normal but he keeps going outside and trying to move his bowels but is having no luck.  His diet has not changed in any way and I have given him 3 teaspoons of milk of magnesia in the last 24 hours and walked him frequently.  Should I be worried or what should my next step be?

Answer
You might want to give him some olive oil and then mix some wheat bran into his food with some water. He needs fiber and water to move things along.
Some dogs get very impacted and have to be manually cleaned out at the vets.
If that is the case then afterward you need to re-evaluate his diet and put him on a better diet with more fiber in it for his age.
I recommend Science Diet Senior for a dog like him.
It is high in fiber but low enough in calories to help him stay in shape and to move his bowels regularly.

If he doesn't move them after you give him some olive oil within 12 hours then you need to call the vet.