You love your dog, but every time he comes in for a kiss, it knocks you flat. It's almost enough to make you avoid that sweet little face.

What's the reason for your pet's horrendous dog breath? It could be something he ate, but chances are there's an underlying medical condition that may warrant a trip to the vet for a little investigation. Here's a look at some of the more common causes for canine halitosis.

Periodontal disease

Periodontal disease, or gum disease, is the primary cause of bad breath in dogs and the most common health problem that vets find in adult dogs and cats, according to the American Veterinary Medical Association (AVMA). By age 2, an estimated 80 percent of dogs and 70 percent of cats have some form of gum disease, reports the AVMA. Gum disease starts when bacteria in the mouth forms plaque that sticks to the surface of the teeth. Plaque hardens into tartar, which becomes especially problematic if it spreads below the gumline. In addition to the possibility of tooth loss, if the disease progresses enough and bacteria enters the bloodstream, it can cause damage to your dog's heart, liver and kidneys, according to the American Veterinary Dental College.

You can try to prevent gum disease by brushing your dog's teeth daily — or at least several times a week — using doggy toothpaste and a toothbrush. You may want to ask your vet about other preventive measures, such as rinses or special chew toys, that may help ward off problems. Your vet also may want you to bring your dog in on occasion for professional cleanings, where your dog will be sedated while the plaque and tartar is scraped away and his teeth are polished.

Teething

puppy chewing on the corner of a colorful pillow Puppies can have bad breath because of teething. (Photo: Jolanta Beinarovica/Shutterstock)

Although puppies usually have the sweetest breath, occasionally it can get a little rank. Blame it on bacteria that sits at the gumline while baby teeth are pushed out by newly erupting adult teeth, says VetStreet.

Diabetes

Dogs with diabetes can have breath that smells distinctly like nail polish remover, says Prevention. It can also smell unusually sweet or fruity, points out WebMD. If your pet has diabetes, you may notice he's drinking and urinating a lot more, maybe even having accidents in the house. Other symptoms include sudden weight loss and an increased appetite, as well as behavior changes like irritability or sleeping even more than usual. If things seem out of the ordinary, head to your vet for blood and urine tests for a diagnosis.

Liver problems

If your dog has exceptionally strong, foul breath and also has other specific symptoms including vomiting, appetite loss and a yellowish color in his gums or eyes, that could be a sign of liver disease, reports WebMD. Your vet can test for problems.

Kidney disease

Dog breath that smells somewhat like urine could be a sign that your dog may have kidney disease, according to WebMD. Other symptoms include changes in weight and appetite, drinking or urinating more or less, and behavioral changes such as listlessness or depression.

Sinusitis or rhinitis

You know how when you get a cold or your sinuses get stuffed up, you breathe through your mouth, making your mouth dry out and your breath take a hit? The same thing can happen when your pet's upper respiratory tract or sinuses are inflamed. It can cause your dog to breathe through his mouth, reports Merck Manuals. The result can be some stinky breath.

Gastrointestinal issues

Stomach and digestive problems, such as the enlargement of the esophageal tube, which goes from the throat to the stomach, could be the reason for oral odors, says PetMD. Talk to your vet if you notice any other unusual symptoms, particularly changes in appetite, nausea or vomiting, or changes in stool.

Icky diet

Every once in a while your dog will decide something absolutely disgusting is delicious. Maybe that's cat poop or garbage, but the results (other than an upset stomach) can be incredibly foul breath. It's usually temporary unless your pet has a chronic love for gross things. In that case, latch the litter box and the garbage can.

Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.