Onions, garlic, chives and leeks are kitchen staples that belong to the Allium family. They contain a compound, N-propyl disulfide, that dogs cannot properly digest.

Ingestion can damage red blood cells and lead to their destruction—a condition known as hemolytic anemia.

Symptoms of this condition may include weakness, vomiting, breathlessness and an increased heart rate.

A human favorite, avocado should be kept off your dog’s menu. The skin, leaves and seeds of avocados contain high levels of persin, a toxin that can cause upset stomach, vomiting, diarrhea and even myocardial damage in dogs.

The interior, or flesh, of avocado contains less persin, and small portions are generally not harmful to dogs. However, the flesh of avocado is high in fat and calories. Consuming too much can lead to an upset stomach, weight gain and pancreatitis in dogs.

Moreover, the large pit can be a choking hazard or lead to an obstruction in the gastrointestinal tract if swallowed.

Learn more about dogs and avocado.

Grapes and raisins can cause acute kidney failure in some dogs, though research has yet to determine exactly which substance in the fruit causes this reaction or why it only happens in some dogs, explains Dr. Fiorenza.

“I recommend that clients do not feed their pets grapes or raisins under any circumstances, and that they treat it as a poisoning if ingestion occurs,” says Dr. Fiorenza. “Veterinary care should be sought immediately.”

Learn more about dogs and grapes and raisins.

Stone fruits such as cherries, plums, apricots and peaches pose a two-fold risk to dogs: First, the pits contain cyanide, a potent poison that—when cracked open and consumed—can cause rapid breathing, excessive panting, dilated pupils and shock. Second, swallowing the pits can lead to choking or obstruction of the gastrointestinal tract.

The fleshy parts of these fruits are generally OK for dogs to consume, but it’s safest to avoid them altogether, as consuming even a small piece of the pit can be harmful.

In general, nuts aren't a good snack for dogs. Their high fat content can lead to digestive issues, weight gain and pancreatitis when consumed in large amounts. Additionally, given dogs' tendency to gulp or gobble their food, whole nuts can present choking hazards or lead to intestinal obstructions.

Specific nuts pose even greater threats:

• Both macadamia nuts and black walnuts harbor compounds that are toxic to dogs. The toxic compounds within these nuts are unknown, but it takes only a few nuts to endanger a small- to medium-size dog. Ingesting these nuts can result in symptoms like vomiting, muscle weakness, tremors, fever and seizures.
• Almonds are also off limits.

Other nuts, like peanuts and cashews, can be given very occasionally in small amounts (1-2 nuts). Just be sure they are plain (no salt, garlic seasoning, etc.).

Chocolate—whether it’s milk, dark, bittersweet, white or ruby chocolate—contains ingredients that can be toxic to dogs. The two ingredients of concern—theobromine and caffeine—can cause stomach upset in smaller doses, and heart problems, seizures and even death in higher doses, Dr. Fiorenza says.

Other caffeine-containing beverages such as coffee, caffeinated tea and energy drinks should also be avoided.

Learn more about dogs and chocolate.