They are generally said to be safe.
What the Experts Say
American Kennel Club
According to the American Kennel Club, feeding dogs cranberries is relatively safe to do.
They stress that the most important thing when feeding cranberries to your dog is moderation. They warn that too much may result in an upset stomach.
They say that both fresh and dried are fine, although again they warn that even though eating dried cranberries are okay, raisins are not. Both actually look alike so that’s why they stress that point.
Cranberry juice on the other hand they warn is risky. They say that many cranberry juice products sold in the supermarket are actually blends of different juices, with the most common other juice mixed in being grapes.
Grape juice can be problematic for dogs and anything with lots of sugar in it (which grape juice has), or lots of sugar added, should be avoided for your loving pet. This means that dried cranberries should be even more limited than fresh ones due to the added sugar in the Craisins.
When it comes to cranberry sauce, they say that it’s generally okay in small quantities but again caution about too many other ingredients, like sugar, alcohol or anything else that may upset the dog’s stomach, being present.
Remember that cranberry sauce is a concentrated form of the berry itself which is why smaller quantities are advised.
As for urinary tract infection treatment? They warn against that too. They say there is no evidence to support the idea that feeding your dog cranberries will help with their UTI.
In fact one veterinarian goes on to say that feeding too many cranberries to your dog may result in calcium stones being formed in the bladder, which would only exacerbate the existing problem.
YOUR BEST BET
The advice given above from the American Kennel Club is echoed by most of the other experts I found on the subject.
You’ll see other websites claiming to be authorities on dog nutrition which echo the same recommendations as above, but then go beyond that to saying that cranberries are good for gut health, bladder health and their immune systems etc.
That may be true some degree, but just like with humans there is a lot more to good health, and the health puzzle altogether, that go into changing your body or your dog’s body fundamentally for the better.
I hope this helps.