Guide to Pet Medications: Antidepressants

Anti-anxiety medications (antidepressants) are some of the most frequently prescribed drugs in human medicine. These medications also serve an important purpose in veterinary medicine. When prescribed appropriately and judiciously, these drugs can help treat a wide array of behavior issues and can make our pets healthier and happier. Behavior problems in dogs and cats can include aggression issues, generalized anxiety, obsessive-compulsive behaviors (such as spinning around in a circle or licking themselves raw in one spot), elimination disorders, and phobias such as thunderstorms, fireworks, or transport.

How Do Antidepressants Work?

Serotonin is a neurotransmitter (a chemical messenger in the brain) that is largely responsible for whether we are happy or sad.  As simplistic as it sounds, we know that low serotonin levels in people are associated with irritability, hostility, and loss of impulse-control. Higher levels are associated with calmness, confidence, and restraint.  We also know that increasing available serotonin levels helps the brain fight depression and anxiety.

This article will focus on two major drug groups which work by altering serotonin levels in the brain: tricyclic anti-depressants (TCA’s) and selective serotonin reuptake inhibitors (SSRI’s). Typically, a nerve cell in the brain will release serotonin so that it can spread its chemical message, and then recycle it by sucking it up - like a vacuum cleaner - so that it can be released again later. TCA’s and SSRI’s basically prevent “reuptake” of serotonin by the nerve cell. This means that they help keep the existing serotonin around longer so it can be more effective at spreading its message.

Why Do Antidepressants Work?

Many behavior problems in pets feature anxiety as an underlying problem. If you treat the anxiety, you can treat the behavior problem. But the real reason this works is that lowering anxiety allows the pet to learn a new, more desirable behavior to replace the old, undesirable one. Pets cannot learn in the presence of anxiety.

For the same reason, treating with medication alone will not solve the behavior problem. Simply lowering the pet’s anxiety does not teach them any new behaviors! The veterinarian prescribing the medication should also prescribe a behavior modification protocol (series of training exercises) so that the pet can re-learn the proper response to a situation.

What Happens Next?

1. First your veterinarian will rule out potential medical causes of the behavior problem.  
2. Next she will diagnose the nature of the behavior problem by taking a detailed behavior history. The diagnosis is critical because there are many different treatments and training protocols.  
3. Pre-treatment laboratory tests should be performed, such as a blood panel and a urinalysis. These will often be repeated once the pet has been taking the medication for a few weeks.  
4. An appropriate behavior modification program is designed and might be tried first alone, without any drugs.  
5. An appropriate medication may be added to the program and is often continued for at least 4-6 months minimum.  After the desired affect is achieved and maintained for this length of time, some animals can be tapered off but many will need to remain on the medication for life.  
6. Be patient!  Many of these medications will not reach effective levels in the blood until the pet has been taking them for 4-6 weeks, so do not discontinue the drug early just because you don’t think it is working.

What If the Antidepressant Isn’t Working?

• There are several drugs in these categories and your pet might have to try more than one prescription in order to find the right one for him. Ask your veterinarian about trying a different drug or altering the dosage before giving up.

• A common cause of treatment failure is when medication is used as a stand-alone therapy, without behavior modification. 

• Never stop the medication cold-turkey!  Always ask your veterinarian how to safely taper the drug dose down until you are no longer giving it.

Common tricyclic antidepressants include amitriptyline (trade name Elavil) and clomipramine (trade name Clomicalm or Anafranil). Common selective serotonin reuptake inhibitors include fluoxetine (trade name Reconcile or Prozac), paroxetine (trade name Paxil), and sertraline (trade name Zoloft). 

Possible side effects of these drugs include decreased or increased appetite, lethargy, gastrointestinal upset, dry mouth, decreased tear production, urine retention, and cardiac arrhythmia. SSRI’s are associated with a slightly lower risk of side effects compared to TCA’s. 

Contraindications and Interactions with Other Drugs

Your veterinarian may not prescribe these medications if your pet has a known hypersensitivity to SSRI’s or TCA’s, is diabetic, is suffering from severe liver disease, or has a seizure disorder.

DO NOT use if your pet is also using a monoamine oxidase inhibitor (MAOI). Common MAOI’s include amitraz and selegiline.  Amitraz is found in anti-tick treatments and collars, as well as Promeris, the topical flea and tick preventative (dog formulation). Selegiline is a drug used to treat canine cognitive dysfunction.