Fluoxetine is an antidepressant in a group of drugs called selective serotonin reuptake inhibitors (SSRI¿s). Fluoxetine affects chemicals in the brain that cause depression, panic, anxiety, or obsession-compulsion. Fluoxetine is a prescription medication used in dogs and cats for the treatment of canine aggression and obsessive-compulsive disorder. Fluoxetine is available as 10mg tablets and 20mg capsules. The usual dose is dependant on the condition being treated and the animal's response to treatment. It may take up to 3 or 4 weeks before the medication becomes effective.