Metamucil (Psyllium) is a human-use treatment for constipation. Is it safe for cats? There is some confusion on the internet. The simple answer is that it is not FDA approved for cats and dogs (in the US) as far as I am aware but that it is safe for cats provided precautions are taken and ideally veterinary advice received because there are dangers in self-diagnosing cat constipation. What if there is an obstruction? Metamucil would be an inappropriate treatment if this were the case.
Also your cat might need prompt intestinal evacuation. Only your vet can decide that. Maybe a X-ray needs to be taken to diagnose the cause of the constipation. It might be oversimplifying things if a cat owner assumes that their cat is constipated and seeks a quick home remedy.
My research indicates that Metamucil can be taken with mineral oil. Metamucil passes through the intestine undigested. It makes the stool bigger and softer by absorbing water which stimulates bowl movement. Psyllium is a plant used to add dietary fiber to a diet.
Free access to plenty of water is a necessity as taking it without adequate fluids can lead to a blockage. My careful research indicates that veterinarians approve Metamucil for cat constipation but no doubt the advice varies.
Pumpkin and olive oil are alternatives and may even be better. Human laxatives can be used too but you have to see a vet about dosage etc..
You administer Metamucil by mixing it with wet food but dosage advice should come from your vet.
It is worth remembering that an overly passive, static cat who is fed dry cat food constantly is more likely to be constipated than an active cat on wet cat food (canned and pouches). A wet food diet of itself should help prevent constipation. Prevention is better than a cure as the saying goes.
Elizabeth Hodgkins DVM (Your Cat) promotes the health benefits of wet cat food over dry. “Problems with constipation often disappear..” and…”when cats change from a high-carb diet [dry cat food is high-carb] to a low-carb one [there is] an astonishing physical rejuvenation…”
Sources: various veterinary websites and forums and as mentioned.