Why is my neutered cat suddenly aggressive towards his twin?

You should have him checked, or rechecked, by your veterinarian. Neutering typically does not result in more aggressive behavior. Additionally, any sudden behavioral change can be due to illness or injury. Once medical causes have been ruled out, you can focus on behavioral issues. Consider a pheromone product, such as Feliway Spray or plug in diffusers, to help decrease anxiety or aggression. Provide additional litterboxes, bowls, toys and bedding to prevent resource protection. You may have to separate them for a week or two then gradually reintroduce them while employing the measures described above.

It takes 4-6weeks after neutering before the hormones and related behavior subsides. Be aware however that not all bad behavior is hormonal and some cats will never be friends and many need to be segregated. Calming diets and supplements such as kalm aid can help alongside the feliway

Hi and thanks for using Petco Pet Education Center, formerly Petcoach! Sounds like Prince is still experiencing some signs associated with the male hormone testosterone. The testosterone levels drop after the neuter and should be decreasing now. Hopefully this behavior will ease up as the remaining testosterone gets out of his system. If things don't resolve or his signs worsen, I would advise that your veterinarian take a look at both of them to assess any problems. I hope this gets better soon.

Thank you for submitting your question regarding your cats. I suspect that the carrier ride and the surgery were stressful experiences for both cats. This can cause a cat to act out aggressively. You may want to keep the cats separate from each other for a few days to let them calm down. Then re-introduce them slowly and only when they can be fully supervised. Things should go back to normal between them, but it may take some time. I hope this information helps!

I'm sorry you're having this situation!  It increases the stress for everyone when inter-cat relationships are aggressive.   While uncommon in neutered males, sexual aggression is still possible. Here is an interesting article from Tufts university regarding this issue.     I haven't personally run into this in my practice, but I have heard of Boar Mate spray being recommended.  Sometimes anti-anxiety medication such as fluoxetine can also be helpful when given to the aggressive cat.   Talk to your veterinarian about specific recommendations for use. http://www.tuftscatnip.com/issues/22_1/feature/Understanding-Feline-Sexual-Aggression-222-1.html

Thanks for using Petco Pet Education Center, formerly Petcoach! On the whole, I’m not worried about this yet. It can take a couple weeks to feel completely normal after a neuter, especially in adult males. If you have pain medication still, go ahead and give it as even mild pain can cause some cats to be a little crankier than usual. Sometimes, a long trip to the vet or anesthesia seems to disturb cat relations and anxiety is more of a problem. If this behavior is not improving with pain medication, or persists for another week or more, consider these anxiety-reducing home helps. -Feliway- calming cat pheromone- comes in many forms from pet stores or online. 

- Adding a litter box. Studies show that the ideal number of litter boxes for a home is #of cats +1 . This is regardless of how clean you keep his current litter box.

 -making sure he has plenty of “high space”. The safest (and therefore least anxiety-inducing) place for a cat is up. If you don’t already, get a cat tree or put up a shelf or two that he can go perch on. Best wishes to you and Gus!

If the two females cats are not spayed I suggest you do that as soon as possible. That could sure be the reason for the tension. Not only that but if the females are not spayed and are going outside they will get pregnant guaranteed as soon as they go into heat. Usually the first heat cycle is around 6-7 months (so now). I also suggest you try some Feliway products in your home. These are basically "happy cat pheromones" that help to de-stress and relax cats. It works very well. Here is a link to their website. You can buy their products at any large pet store or online. https://www.feliway.com/us#redirected Hope this helps. Best wishes.