Not a poison — still not a good table treat
Hazelnuts sit in the safer half of the nut bowl for dogs: unlike macadamias, which genuinely are dangerous to them, a plain hazelnut will not poison a dog who snaffles one. That reassurance has limits, though, because the risks here are mechanical and digestive rather than toxic. A hazelnut is round, hard and exactly windpipe-sized for a small dog, and it is rich enough that more than a taste can leave a dog with a greasy, uncomfortable stomach. So the honest answer is a qualified yes to the nut itself, served properly — and a firm no to the holiday-table free-for-all.
Whole nuts, shells and the gulping dog
Dogs do not shell nuts thoughtfully; they crunch, gulp and compete with the floor. A whole hazelnut swallowed by a small or enthusiastic dog can lodge in the throat, and even one that goes down can trouble a narrow gut on its way through. The shells you discard are no safer: cracked edges are sharp, the pieces do not digest, and a dog hoovering the shell pile under the table collects exactly the fragments you would least choose for him. During a cracking session he is better out of the room or settled with his own chew at a distance, with a floor sweep before he is allowed back to his usual hopeful position under the table.
If he gets a taste, chop it fine
Choose plain hazelnuts — unsalted, unroasted in flavourings, and absolutely not the chocolate-coated or spiced holiday kinds, since chocolate changes the conversation entirely and becomes a vet call if he has eaten it in quantity. Chop one nut finely and sprinkle the crumbs over his food or offer them from your palm; that is a taste, which is all a food this fatty should ever be for him. Nuts that have sat in a bowl for weeks, smell musty or show any mould go nowhere near him: mouldy nuts can carry toxins that are a far more serious matter than the nut itself, and a rancid bowl is a bin job, not a dog treat.
The dog who already raided the nut bowl
Count what you can and watch him for the rest of the day. A couple of plain nuts in a large dog usually pass with nothing worse than a rich-food stool tomorrow; a small dog who has eaten a pile of nuts, shells included, deserves closer watching. Choking or gagging at the time, repeated vomiting, straining, a sore belly or sudden lethargy afterwards are all vet-call signs rather than wait-and-see ones. Rich fatty raids can also flare up a sensitive stomach a day later, so keep his following meals plain and modest while his system settles.
Holiday food risks multiply because guests feed dogs generously and nobody tracks the total. Putting 'no nuts, floor swept after cracking' on his care list in KinsLe through Care Teams turns a house rule into something the whole table can see, and the duty marked done each evening tells you the sweep under the nut bowl actually happened.