Every now and then our canine companions get a little backed up and we have to deal with their constipation. As long as this isn't a frequent condition and there are no other signs or symptoms of illness, such as fever, vomiting, loss of balance, lethargy or anything else that causes you concern, you may want to try adding some fiber to their diet and hope everything comes out ok
Good sources of high fiber tolerated by your friend would be a tablespoon or so of canned pumpkin (100% natural pumpkin, not the pumpkin pie mix); or sprinkle a couple spoons of wheat bran (or brewer's yeast), offer them a couple dried plums (aka: prunes), dried figs or even a bit of unsweetened dried coconut.
Call your vet for advice about how much Metamucil you can offer, based on your dog's health history, size/weight. Generally a dog this size, about ½ to 1 teaspoon every 8-12 hours.
Milk of Magnesia = 1 teaspoon per 5 lbs of body weight.
Making sure your dog gets regular fiber so this doesn't happen again (or is less likely to occur) is important, just as it is for all of us. Certain kibbles address this with their ingredients and you may want to switch brands, or simply contribute to the dog's daily diet with the items mentioned above.
This link provides more information about this http://www.dogpro.com/constipation-in-dogs.html
Don't forget, if constipation continues more than a couple days or there are any other symptoms of concern, there may be a blockage or other serious condition that requires veterinary intercession early in order for chances for survival to be good.
Whether there's a foreign object lodged in their intestines, tumor, even a fecal ball, peritonitis may result. Untreated, it is life threatening.
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Good luck and keep up the good work !