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By Wendy C. Brooks, DVM, DipABVP
Educational Director,

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Fluoxetine (Prozac)

Brand Name: Prozac

Available in 10 mg, 15 mg, and 20 mg tablets; 10 mg, 20 mg, 40 mg, and 90 mg capsules; oral liquid


Anxiety, obsessive-compulsive disorder, and depression are common problems for humans. Because the development of drugs that alleviate these conditions is ongoing, there are currently many such medications on the human market. One of the more popular classes of anti-anxiety medications are the selective serotonin reuptake inhibitors, called SSRIs.

Serotonin is a neurotransmitter in the brain associated with, among other things, mood elevation and reduced aggression. Increasing serotonin in the brain means less anxiety and a happier attitude. By inhibiting the brain’s system for removing used serotonin, SSRIs cause serotonin to linger, lasting longer. The more serotonin we have in our brains, the less anxiety, obsession, and depression we get.

Eli Lilly, a prominent pharmaceutical company, tested many related compounds and released fluoxetine as the most specific “selective serotonin reuptake inhibitor” of all of them. Fluoxetine, marketed under the brand name Prozac®, was not quite the first SSRI on the human market but quickly became the most popular.

It was not long after that that fluoxetine found its way into veterinary use for animals with anxiety, compulsive behavior, and other behavior issues. When low-cost generic fluoxetine became available, its use increased in the veterinary market and in 2007 Eli Lilly released a version of fluoxetine specifically labeled for animal use and sponsored a behavioral program to go with it. Widespread use of generic human products supplanted the brand name veterinary product for a time but PRN, another pharmaceutical company, has reintroduced not only the brand name product but also the B.O.N.D. behavior modification program that improves its efficacy (see below).

How this Medication is Used

Fluoxetine has been used in many animal behavior issues: inappropriate urine marking in both dogs and cats, separation anxiety, compulsive chewing, circling, and self-mutilation, even aggression. 

It should be noted that the treatment of behavior disorders requires training in addition to medication. When medication and behavior modification were combined, 42 percent of dogs with separation anxiety showed improvement by the end of the first week and 73 percent were improved within 8 weeks. These statistics were obtained using the B.O.N.D. training program designed for use with brand name Reconcile®. For more information, visit Obviously, there are a number of training techniques for use in the management of separation anxiety; the point is that best results are obtained when fluoxetine is combined with training. 

Fluoxetine is usually given once daily.

Side Effects

The most common side effects of fluoxetine in dogs and cats use is drowsiness or lethargy, but the opposite is also reported: hyperactivity, panting (dogs), irritability, and/or insomnia. Appetite reduction has also been described. Some dogs (approximately 1 dog in 3) will show some degree of weight loss which should not exceed 15% of their original body weight.

It is important to understand that whenever an anti-anxiety medication is used, the phenomenon of disinhibition is possible. What this means is that an animal’s inhibitions about aggressive behavior may be reduced when anxiety over the consequences of such behavior is removed. An animal that was not previously aggressive could potentially become aggressive.

Some patients will experience an upset stomach with this medication.

Interactions with Other Drugs

Serotonin syndrome is a potentially dangerous situation that can result when serotonin levels get too high. Elevated heart rate, tremors/shivering, dilated pupils, difficulty breathing, elevated body temperature, hyperactivity, and/or high blood pressure can all be signs of serotonin syndrome. Fortunately, the development of serotonin syndrome generally requires a combination of at least two serotonin-increasing drugs and rarely happens spontaneously with one medication but it is important to be aware of the symptoms as the high blood pressure can be life-threatening if severe enough. Using MAO inhibitors in conjunction with fluoxetine could create serotonin syndrome. MAO inhibitors are rarely used in veterinary patients with the exception of selegiline, a drug used for cognitive dysfunction in dogs; amitraz, an anti-parasite topical used in several tick control products (see our flea and tick product comparison chart); and in Mitaban® dip used against mange. Fluoxetine should not be given in conjunction with a MAO inhibitor such as selegiline or amitraz.

The use of buspirone, tramadol, clomipramine, or amitriptyline with fluoxetine can also increase the risk of serotonin syndrome.

Cyproheptadine, an appetite stimulant, may decrease or even reverse the effect of fluoxetine.

Diazepam and alprazolam may have stronger effects if used in conjunction with fluoxetine.

Fluoxetine should not be used in combination with drugs that could increase the likelihood of seizures (such as acepromazine).

Insulin requirements may be altered when taking fluoxetine.

Concerns and Cautions

This medication is best not used in patients with diabetes mellitus or with seizure disorders.

Fluoxetine lasts a long time in the body. If planning to discontinue fluoxetine, a tapering course is not necessary unless the patient has been taking fluoxetine for > 8 weeks. In that case, tapering the dose over a couple of weeks is a good idea.

Fluoxetine and MAO inhibitors should not be given together and a "wash out" period is needed between them. If one wishes to begin an MAO inhibitor (selegiline for cognitive dysfunction or amitraz for parasite control) in a patient presently on fluoxetine, a five-week period is recommended between the last dose of fluoxetine and the first dose of the MAO inhibitor. Similarly, if a patient is on an MAO inhibitor and will be beginning fluoxetine, a two-week period is needed between medications. The period is longer in the former incidence because of the long half-life of fluoxetine in the body.

It is our policy not to give dosing information over the Internet.


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