I happened to see this reply on another site someone asking a similar question. Perhaps there is info here that can help:
If it is happening immediately after you picked him up from being neutered, and he has just come out of his light anesthesia, and “craziness” means acting a little disoriented, drunk, carsick, agitated (growling, banging around in the carrier, howling and crying), or like he doesn't recognize you, this could have to do with the car trip or cat carrier or the handling by strangers or the actual anesthesia, but I would expect it to pass quickly, as the rest of the anesthesia wore off and he began to realize he was back safe at home. They do not use as deep anesthesia when neutering males as they do when spaying female cats, because spaying is a bigger surgery where there is a long abdominal incision through several layers, where organs are removed, sutures or staples are placed, the surgery takes longer, and recovery from the anesthesia is a slower process than with males. As for post-op distress due to a pain level, some clinics will send their patients home with liquid post-op pain medication to take by mouth, in prepared needle-less syringes that contain the proper dose, or else as an injection that is given by the vet before they return the cat to you. More often this is done for the females than for the males, but even they don't usually need more than about about 2-3 days worth, if they have it at all. if they do give you oral meds for your cat, you must give them to your cat as ordered, because the vet has decided they were needed. Not only will the pain and inflammation be reduced but vital signs will be more stable, the cat will be more relaxed, there will be less risk of infection or worrying at the incision, the cat will rest better, and overall healing will be quicker. So if you should suspect that your cat is acting “crazy” because of uncontrolled pain, do not hesitate to advise the clinic of that ASAP.
Both sexes can act “crazy” afterward, as you say, and disoriented and defensive when coming out of it, just like postop people. There can also be a buildup of anxiety and stress from preoperatively having been unexpectedly put into a cat carrier, then a car, then a cage, then prepped and secured on an operating table, getting mind-altering medication and then afterward reversing the process. There is also the possibility that they are having more pain than was expected, and again, if that is the case, you need to let the vet know. And after all, the cat's understanding of events is at a cat's level. So if they are a little out of sorts, I think it is understandable.
If they are acting truly “crazy,” at risk of injuring themselves or someone else, or worrying you about their safety, then I think there may be the possibility that they are having an unexpected/ adverse reaction or side effect, and in that case I would not hesitate to contact the veterinarian or clinic that performed the surgery, ASAP. They might advise you to bring the cat back or come pick uo another medication, or to hydrate the cat (who was fasting preop), or to do something else that is familiar and soothing. I don't know the protocol since none of the cats I have ever taken for neutering has ever acted any way but a little confused or unsociable afterwards, and I always thought that it was just kind of an anesthesia hangover. Personally what I would do in the meantime with a “crazy” postop male would be to seclude the cat, not risk irritating him further by even talking to him or petting him, but just let him close his eyes and relax, either back in a covered but well-ventilated cat carrier or in a darkened bedroom, where he was getting plenty of air into his lungs and with the door closed and nobody else in the room, and just give the cat a couple of hours and a bowl of water in case he got thirsty, and let him sleep it off. This is what I would do unless I was advised to do something differently by the professionals. I would not handle the cat because of its maybe-tender surgical incision and also because of the possibility that I might unexpectedly get bitten by a confused and startled cat who was normally very sweet but who was currently disoriented and defensive.
On the other hand, if the cat's “crazy” behavior started to become unusual a longer period of time after it had been neutered, I would definitely talk to the vet. Frankly I have never heard of this happening. Neutering is considered minor surgery. But if it happened, I would have the healing of the incisional area checked for infection, inflammation, tenderness, swelling, retained surgical materials or wound dehiscence. I would try to find out if there had been any complications, like growths, found and treated at surgery, or perhaps any drama involving another cat in the holding area or the room where surgery was done, or any possible hormonal imbalances, or exactly how often something like this happens and what course it usually takes, because I don't know much about it from personal experience.
I suppose it's possible that your cat is just having trouble trusting you again since you recently abruptly took it to a surgical clinic and had strangers do something to its body while it's consciousness was altered. But PLEASE rest assured that again, I have never known this to happen; the process of spaying or neutering a cat does not change its personality into that of a crazed animal, and you do not risk that by having your cat sterilized. In fact, the vast majority of the time, sterilization makes the cat even sweeter, cuddler, calmer, more affectionate, nicer to other cats, and more inclined to stay at home with you because they are no longer victims of their hormones. If your cat does seem to be having psychological aftereffects, it may take a little while for the cat to trust you completely again. I don't know for certain, but this is what makes sense to me: I would reassure my cat by remaining the consistently trustworthy person they know, continue to treat my cat the same as I always had, lovingly and patiently and with a lot of talking and petting and interacting, and I think things will pretty quickly return to normal as the memory fades.
Your vet is always the best source of information about this and everything regarding your cat, and he or she should always be kept informed of any developments with your cat’s health in order to take the best possible care of him. So they needs to know everything. Speak up for your cat.
