While doing some research about dysphoria I stumbled onto this excellent article by Stephanie Krein, DVM, DACVAA of MSPCA Angell Animal Hospital in Boston.

Pain Versus Dysphoria

Dysphoria is a profound state of unease or dissatisfaction accompanied by anxiety or agitation. Dysphoria can be very frustrating for the animal and the staff involved, and at times can be difficult to identify or treat. Clinical signs seen in a dysphoric animal include vocalization, panting, or struggling.

Dysphoria is commonly caused by the very treatment we are giving in attempts to remedy the pain an animal is experiencing. Opioids are commonly known to induce dysphoria in some animals and some species (cats more often than dogs). Opioid induced dysphoria is accompanied by an agitated animal that was recently treated for pain with opioids or an animal that does not respond to human contact.

Dysphoria caused by opioids can often be reversed with a low dose of a reversal drug such as butorphanol (a full mu antagonist and kappa agonist) or naloxone. An animal that is dysphoric from opioids should not be withheld pain medications and should be treated with a different class of analgesic or a different class of opioids. Often sedatives can be helpful in dysphoric animals. It is important though when sedating an animal to first ensure that their pain in being managed appropriately. 

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