484 Canine Sports Medicine and Rehabilitation
These drugs are FDA scheduled drugs, and
come as separate drugs or combined with
acetaminophen. However, in one study of dogs
undergoing stifle surgery, oral acetaminophen
- hydrocodone given without other analgesics
was considered to render inadequate postoper-
ative pain control (as was tramadol in the same
study; Benitez et al., 2015b). Therefore acetami-
nophen + opioid must be considered strictly as
an adjunctive analgesic medication to a com-
prehensive pain management strategy, and not
as a stand‐alone agent.
Partial mu agonists
Buprenorphine (Buprenex®) has greater affin-
ity than morphine on the mu receptor and it has
a ceiling effect. However, it only partially binds
to the receptor, meaning it cannot have the
analgesic punch of a pure or full mu agonist.
Wide variation of onset and duration appears
to exist in the dog, even when administered
intravenously (Krotscheck et al., 2008), and may be
dose‐dependent. Compounded sustained‐release
buprenorphine formulations are available for
use in dogs, but they are not FDA‐approved
products nor are their kinetics well established.
Anecdotally, dogs seem to experience signifi-
cant and prolonged sedation at doses recom-
mended by the compounding pharmacy.
Oral transmucosal (OTM) bioavailability is
modest at best in dogs (~40%; Abbo et al., 2008),
and the volume required makes OTM adminis-
tration mostly impractical. Commercial
buprenorphine patches (e.g., BuTrans®) have
compared favorably in dogs to i.v. buprenor-
phine in a thermal threshold model (Pieper et al.,
2011) and pain scores post‐ovariohysterectomy
(Moll et al., 2011).
Kappa agonist/mu antagonist
Butorphanol will weakly block the mu‐opioid
receptor, but its kappa agonism will promote
the release of inhibitory neurotransmitters such
as GABA. It has a ceiling effect and a very short
duration of, and effect on, visceral analgesia in
the dog (~30–40 minutes; Sawyer et al., 1991;
Grimm et al., 2000). This makes it a poor choice
in this species for any kind of significant or pro-
tracted pain states. It does have utility in select
situations such as when administered in a CRI,
and as an adjunct with other medications such
as alpha‐2 agonists.Mu antagonist
Naloxone (Narcan®) is a potent mu‐opioid
receptor antagonist, traditionally used to
achieve rapid and complete reversal of opioid
overdose or severe ADEs. However, micro-
doses (as low as 0.01–0.05 μg/kg i.v.) have been
used to improve the analgesia provided by
buprenorphine (La Vincente et al., 2008) and to
minimize adverse effects of i.v. morphine
(Cepeda et al., 2004).
New developments in other long‐acting opi-
oids including novel formulations and delivery
systems may overcome the need for, and limita-
tions of, opioid intravenous CRI, patches, and
oral administration. Opioids for all their effec-
tiveness may create clinical challenges as well. In
the acute setting, opioid‐induced dysphoria, nau-
sea and inappetance, hypothermia, and, in the
extreme, hyperalgesia and respiratory depres-
sion may be encountered (McNicol & Carr, 2007).
Having strategies for recognizing and counter-
acting these signs will minimize complications.
With chronic use in humans, the most commonly
reported opioid ADE is constipation. New
peripherally acting mu‐opioid receptor antago-
nists (POMORAs) alvimopan (Entereg®) and
methylnatrexone (Relistor®, Naloxegol®) permit
the central analgesic effect of opioids but block
their effect on GI motility (Gevirtz, 2007). These
POMORAs have not been clinically evaluated in
dogs, and although potentially helpful, their use
may be limited due to the generally short
duration of opioid use in animals.Tips for use
Preoperatively, opioids are combined with an
anxiolytic (tranquilizer/sedative) to create a
profoundly relaxed, stress‐free, and anesthetic‐
sparing state. The choice of opioid, route, dose,
and duration of administration is dependent
upon clinical preferences and patients’ individ-
ual needs.
A common postoperative adverse effect of opi-
oid use is nausea and/or inappetance. The preop-
erative administration of maropitant (Cerenia®)
has been shown to speed recovery of appe-
tite in dogs having received hydromorphone