A plain dill bite is not toxic, but it is not a treat
Dogs can eat a small piece of a plain dill pickle without being poisoned, because at its core a pickle is a cucumber soaked in vinegar brine. The problem is what the brine does to the cucumber: it loads it with salt far beyond anything a dog needs, along with vinegar acidity that can bother an empty stomach. So the honest answer is that an occasional rinsed slice is harmless for most healthy dogs, while pickles as a regular snack work against his health in a way fresh cucumber never would.
Not all pickles carry the same risk
Dill pickles are the mildest case. Sweet pickles and bread-and-butter styles trade salt for sugar, adding empty calories and sometimes sweeteners that do not belong in a dog. Hot and spicy pickles carry chili and mustard seed heat that irritates the gut. The styles to keep away entirely are those brined with onion or garlic pieces, because both plants can damage a dog's red blood cells when eaten in meaningful amounts, and a dog chewing through several spears also chews whatever shared the jar with them.
Salt is the main long-term issue
A single pickle spear can hold a large share of the sodium a small dog should have in an entire day, and dogs offered salty foods regularly drink more, urinate more, and put extra strain on the heart and kidneys over time. Dogs with heart disease, kidney trouble, or high blood pressure should not get pickles at all, even as a rare bite. Watch for the dog who learns to love salty snacks, because he will start working harder to steal them from plates, lunchboxes, and low picnic tables where sandwiches sit within reach.
If you share, share it this way
Cut a thin slice from a plain dill pickle, rinse it under water to pull off surface brine, and offer just that slice, not the spear. Fresh cucumber is the better everyday swap: it gives the same crunch and hydration with no salt or vinegar, and most pickle-loving dogs accept it happily once it becomes the routine offering. Keep the pickle jar closed and out of counter-surfing range, since a determined dog who drinks brine or eats several whole pickles has had far more salt than a shared slice ever delivers.
When a pickle incident needs a vet
Contact your veterinarian if your dog eats many pickles at once, drinks a quantity of brine, or gets into pickles brined with onion or garlic, especially a small dog. Signs worth a prompt call include repeated vomiting, diarrhea, extreme thirst, disorientation, or pale gums. Tell the clinic which style of pickle and roughly how many, because a sweet pickle binge and an onion-brined one call for different monitoring in the hours that follow.
Lunch rules that survive a busy kitchen
Pickles reach dogs through small kindnesses at sandwich time, one spear from each person's plate. A household note that pickles are a no, with fresh cucumber as the approved swap, gives everyone, including visiting sandwich-makers, the same script. KinsLe helps that script hold through Care Teams, where the people who feed your dog can see which table foods are approved and which salty extras stay on the human side of the counter.