Weaker opioids, such as codeine and tramadol, can be useful for moderate cancer pain. Tramadol has been around for over 15
years and has received attention in recent years regarding its use for chronic pain, both in people and companion animals.
Tramadol is considered a good analgesic for moderate cancer pain in people and is better tolerated than equianalgesic true
opioids.2,3,7,54,55 Recent pharmacokinetic and pharmacodynamic studies in dogs, in addition to the growing body of anecdotal evidence of analgesic
efficacy by various investigators, suggest that tramadol may be safely and effectively used for various pain conditions in
dogs.56-59 Still, few reports exist on the efficacy of tramadol for treating pain in dogs or cats, though studies are ongoing. Tramadol
is a mu-receptor agonist and also has serotonin and norepinephrine reuptake inhibition effects.2 Combining it with other analgesics, including NSAIDs, provides better analgesia.
The main alpha2 agonists are medetomidine and xylazine, which are useful in the context of multimodal and preemptive analgesia. Their analgesic
effects are especially pronounced when combined with other agents such as opioids and ketamine.9,14 Medetomidine is often used in combination with opioids to provide adequate analgesia for moderately to severely painful procedures
and is known to have an opioid-sparing effect. Alpha2 agonists may cause bradycardia secondary to increased vagal tone and are contraindicated in patients negatively affected by
a decreased cardiac output and increased afterload.9,14 They may also cause transient hypertension.
Currently, medetomidine is most commonly used as a preanesthetic or for short procedures requiring mild to deep sedation.
It is most often combined with an opioid, and we recommend a dose of 1 to 6 µg/kg given intramuscularly or intravenously in
dogs and a dose of 2.5 to 10 µg/kg intramuscularly or intravenously in cats. The lower end of the dose range is used for intravenous
Several classes of drugs can be used as adjuvant therapy—local anesthetics, NMDA antagonists, anticonvulsants, tricyclic antidepressants,
aminobisphosphonates, and corticosteroids (Table 3).
Table 3. Adjuvant Analgesic Drugs Used to Treat Cancer Pain in Dogs and Cats
Local anesthetics are valuable in many situations, and, being often used for local or regional blocks, their intravenous,
oral, or transdermal application can also be helpful in certain conditions.4,7,9,14 A good local or regional block may help provide adequate pain control and permit much lower doses of systemic drugs. The
systemic administration of certain sodium channel blockers, such as intravenous lidocaine and oral mexiletine, to potentiate
analgesia from other drugs, is becoming more common in treating cancer pain in people.2,4,7,14 The systemic administration of local anesthetics for analgesia in dogs and cats is relatively recent and may increase in
the near future.
This class of analgesics includes drugs such as ketamine, tiletamine, amantadine, and dextromethorphan. Ketamine has been
in use for many years as a dissociative agent, but its NMDA-receptor antagonistic effect is now considered an important part
of its central analgesic effect.9 It is especially useful when combined with other agents and permits adequate analgesia with lower doses of opioids (sparing
effect) and lower side effects. It can be effective for intraoperative and postoperative analgesia when used at a microdose
in a continuous-rate infusion combined with fentanyl or morphine. Amantadine was initially developed and used as an antiviral
agent against influenza in people and is available as an oral preparation. It also has NMDA-receptor antagonistic activity,
and its use in people as an analgesic for chronic cancer pain has recently increased.2-4 No studies have evaluated the use of amantadine for pain treatment in dogs and cats, but investigations are ongoing.9,11,14