CAUTION! Grapes Are Toxic to Dogs
(also raisins, chocolate, cocoa, onions, garlic, nuts, alcohol, avocados, and others)

Eating grapes or raisins, as well as some other foods, can cause a dog’s kidneys to lethally shut down. No one knows why.

“According to the ASPCA, around 1989, a disturbing trend began to emerge from the AnTox database used by its Animal Poison Control Center: Nearly all the dogs reported to have eaten grapes or raisins developed acute renal (kidney) failure. These cases were noted all across the USA, with the amount eaten varying widely, from over a pound of grapes to as little as a single serving of raisins.

The database showed that dogs who ate the grapes and raisins typically vomited within a few hours of ingestion. Most of the time, partially digested grapes and raisins could be seen in the vomit, fecal material, or both. At this point, some dogs would stop eating (anorexia), and develop diarrhea. The dogs often became quiet and lethargic, and showed signs of abdominal pain. These clinical signs lasted for several days — sometimes even weeks.

When medical care was sought, blood chemistry panels showed consistent patterns. Hypercalcemia (elevated blood calcium levels) was frequently present, as well as elevated levels of blood urea nitrogen, creatinine and phosphorous (substances that reflect kidney function). These chemistries began to increase anywhere from 24 hours to several days after the dogs ate the fruit. As the kidney damage developed, the dogs would produce little urine. When they could no longer produce urine, death occurred. In some cases, dogs who received timely veterinary care still had to be euthanized.

Although it is not known what component of the grapes or raisins causes renal failure in dogs, certain possibilities have been ruled out, including various pesticides, some heavy metals such as zinc and lead, and fungal contaminants. That dogs react in this fashion to both commercially-produced grapes, as well as those grown informally in their owners' back yards, indicates the likely culprit has nothing to do with the growing or cultivation process but is instead basic to grapes themselves.

In other words, all grapes are potentially dangerous to dogs — both grapes in the plump, "just picked" form, and as their dried raisin counterparts, regardless of whether they came from the store or off the neighbor's vine. Don't feed your dog grapes or raisins, and don't leave this possibly enticing foodstuff out where a pet could help himself to them.

This is not to say you need live in fear of your pooch's keeling over dead if he swallows a grape or two. However, if he downs a handful of grapes, or even a smaller amount of raisins, get him to your veterinarian right away. Aggressive treatment with intravenous fluids and close monitoring are his best chance for survival.

Grapes and raisins aren't the only people foods known to be dangerous to man's best friend. Chocolate and cocoa mulch can prove deadly to dogs, as can onions and macadamia nuts.“

References
ASPCA 
National Institute of Health 
SNOPES on Grapes and Dogs 
SNOPES on Cocoa Mulch and Dogs