[GrouchoBarks]

My elderly neighbor is having a real problem with her beloved pup. She lives alone, and the dog is her main company most days. The dog has been having sudden and unpredictable mood swings. One moment, she’s sweet and affectionate, wagging her tail, eager to curl up on the couch with me, and the next, she’s growling, showing her teeth, and acting like a completely different dog. It’s as though she’s two completely separate personalities living inside the same body. My poor neighbor is beside herself and isn’t sure what to do. I wondered if this behavior had a name, and I came across something online called “Jekyll and Hyde Syndrome” in dogs.

Is this a real thing, or is it just internet nonsense? The unpredictability is exhausting and confusing – what causes a dog to shift so dramatically from calm to aggressive? Could something like stress or anxiety be at the root of it? I’m also concerned that this might be more than just behavior. Is there an underlying health issue, like pain or neurological problems, making her act this way?

Has anyone ever heard of Jekyll and Hyde Syndrome in dogs? Any advice on what the next steps should be? My neighbor loves this dog dearly, and I want to help her get through this. I’m concerned that things may get out of hand, and my neighbor could get hurt.

[Emma Braby]

It’s great that you’re looking out for your neighbor and her dog, especially since sudden mood swings in dogs can be a sign of something more serious. The term “Jekyll and Hyde Syndrome” isn’t an officially recognized veterinary or behavioral diagnosis, but the pattern of extreme and unpredictable shifts between calm and aggression is very real and could have multiple causes.

One of the biggest concerns in a case like this is pain or an underlying medical issue. Sudden aggression in a dog, especially one that was previously affectionate, can stem from chronic pain, neurological disorders, cognitive decline (like doggy dementia), or even something as simple as poor vision or hearing loss. If the dog is experiencing discomfort, she may lash out unpredictably, especially when startled or touched in a painful area. A vet visit is the first and most important step to rule out medical issues like arthritis, dental pain, infections, or more serious conditions like brain tumors or seizures.

If health issues are ruled out, then stress, anxiety, or fear-based reactivity could be playing a role. Changes in routine, the owner’s emotional state, or even mild cognitive dysfunction (similar to dementia in humans) can make a senior dog more unpredictable. A dog that becomes confused or anxious may react with sudden aggression, even toward people she loves. This can also happen if she feels cornered, overwhelmed, or unable to escape a situation she finds stressful.

To help your neighbor, start by encouraging her to take the dog to the vet as soon as possible to check for medical issues. We have a guide on the best mobile vets that might be more suitable for your elderly neighbor.

In the meantime, observe the dog’s body language closely and try to identify any triggers. Does she growl when touched in a certain spot? Does she react negatively in specific situations? Keeping interactions calm, predictable, and giving the dog space when needed can help prevent sudden outbursts. If aggression continues after medical causes are ruled out, working with a certified canine behaviorist might help find a safe way to manage her behavior.

Your concern is valid, especially since your elderly neighbor could be at risk if the dog’s aggression escalates. The best thing she can do right now is seek veterinary guidance as soon as possible. If it turns out to be a medical issue, proper treatment could bring her sweet pup back to her normal self. If it’s behavioral, a professional can help find solutions to make life safer and less stressful for both of them.

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