Fluoxetine is in a class of drugs called Selective Serotonin Reuptake Inhibitors, or SSRIs. This medication increases the body’s natural serotonin levels in the brain, which helps pets stay more calm, become less anxious, and can reduce aggressive behaviors. Fluoxetine, a longer-term onset drug, that often causes less sedation compared with faster-onset drugs, typically takes between 4 and 8 weeks to take effect and create noticeable behavioral changes. For that reason, your pet should receive this medication for at least 4-8 weeks before evaluating efficacy, so long as there are no serious side effects. We prefer to start at a lower dose for the first month, then increase the dose once their body has become adjusted to the medication in an attempt to minimize any side effects. The biggest adverse effects are associated with GI upset and can be lack of appetite, vomiting, diarrhea, and/or weight loss. Other signs, although infrequent, include tremors, restlessness, and excessive vocalization.