When your cat becomes unmanageable, such as attacking you, not using the litter box, and even maybe destroying your house, it's easy to just write him off as growing up to be a bad pet. However, cats can suffer from chemical imbalances, just like people. And while we may not like it, sometimes, giving them medication is the best thing for them.
An article on Today.com talks about sweet kitten Coco, who was a good girl until she grew up. Then, said Jackson Galaxy, "She hits a certain age, she snaps. It's just something that's kind of common." She had a chemical imbalance that was contributing to the problem. After they got her on some mood-stabilizing medication, she did much better.
Two drugs that cats seem to respond well to are fluoxetine (Prozac, Reconcile) and clomipramine (Anafranil, Clomacalm). Fluoxetine is a selective-serotonin reuptake inhibitor (SSRI), and clomipramine is a tri-cyclic anti-depressant. Both work to increase the amount of serotonin between synapses in cats' brains, which can reduce aggression and anxiety.
You shouldn't give either of these medications to your cat except under the direction of your vet. The reason for that is human medications can be dangerous for cats. The doses that work for us are way too high for your cat, and you need your vet to help determine his proper dosage.
Also, the problem might not be a chemical imbalance in their brain; they might have something else going on. So when your cat's behavior is out of sorts, make sure you take him to the vet to rule out physical problems. Sometimes cats act out when they're in pain, too, and urinating outside the litter box can be a sign of a urinary-tract infection, crystals, or something even more serious.
Once your vet rules these problems out, then you can discuss the possibility that your cat's behavior is due to anxiety or another chemical imbalance that these medications can help with. Follow your vet's directions very, very closely when administering the meds. If he's prescribed a long-term course, and you worry that shoving a pill down your cat's throat every day will just make things worse, try using pill pockets, or wrapping the pill up in a piece of cold cut. You can also crush it up into powder and mix it in his food if you feed canned food or a raw diet. Ask your vet for advice if these methods don't work.
The bottom line is, you want your cat to feel the best he can. That means being sure that what he needs is mood stabilizing medication, and giving it to him correctly.