Fluoxetine/Olanzapine (Symbyax) Side Effects: What to Expect and When to Call Your Doctor

Fluoxetine/Olanzapine (Symbyax) combines two active medications — an atypical antipsychotic (olanzapine) and a selective serotonin reuptake inhibitor, or SSRI (fluoxetine). Because it contains two drugs working through different mechanisms, its side effect profile is a combination of both. Knowing what to expect — and which symptoms require urgent attention — can help you manage your treatment more confidently.

Important Safety Warnings (Boxed Warnings)

The FDA requires Symbyax to carry two boxed warnings — the most serious type of warning label:

Increased mortality in elderly with dementia-related psychosis: Symbyax is not approved for treating dementia-related psychosis. Antipsychotic drugs, including olanzapine, increase the risk of death in elderly patients with this condition.

Suicidal thoughts and behaviors: Antidepressants including Symbyax may increase the risk of suicidal thoughts or behaviors in children, adolescents, and young adults, especially in the first few weeks of treatment or when doses change. Monitor closely and contact your prescriber immediately if you notice any changes in mood, thoughts of self-harm, or unusual behavior.

Common Side Effects (Occur in 5% or More of Patients)

In clinical trials, the following side effects were reported in 5% or more of Symbyax-treated patients at a rate at least twice that of placebo:

Somnolence/drowsiness (~21%): This is the most common side effect. Taking Symbyax in the evening helps reduce daytime sleepiness. Avoid driving or operating machinery until you know how this medication affects you.

Weight gain (~17%): Olanzapine is one of the antipsychotics most associated with weight gain. It can increase appetite and cause metabolic changes. Regular monitoring of weight, blood glucose, and cholesterol is important.

Dry mouth (~16%): Staying hydrated and chewing sugar-free gum can help.

Increased appetite (~13%): Closely related to weight gain. Discuss appetite management strategies with your provider.

Fatigue/asthenia (~13%): Tiredness and weakness are commonly reported, particularly in the first weeks of treatment.

Orthostatic hypotension: A drop in blood pressure when standing up, causing dizziness. Stand up slowly from a sitting or lying position to reduce fall risk.

Metabolic Side Effects: The Monitoring You Need

Because of the olanzapine component, Symbyax carries significant metabolic risks that require ongoing lab monitoring:

Blood sugar (glucose/HbA1c): Symbyax can cause hyperglycemia (high blood sugar), sometimes severely. Fasting blood glucose testing should be done at baseline and periodically during treatment. Patients with diabetes or pre-diabetes need especially close monitoring.

Cholesterol and triglycerides: Clinical trials showed a mean increase in total cholesterol of 12.1 mg/dL with Symbyax. In long-term studies (at least 48 weeks), cholesterol increases from normal to high occurred in 12% of patients.

Weight: Weight should be monitored at baseline and regularly during treatment. Weight gain can be significant, particularly in adolescents.

Serious Side Effects: When to Call Your Doctor or Seek Emergency Care

The following are rare but serious side effects that require immediate medical attention:

Neuroleptic Malignant Syndrome (NMS): A rare but potentially fatal reaction to antipsychotic drugs. Symptoms include high fever, severe muscle rigidity, rapid heart rate, sweating, and changes in consciousness. Seek emergency care immediately.

Serotonin syndrome: A dangerous buildup of serotonin, especially if Symbyax is taken with other serotonergic medications. Symptoms include agitation, rapid heart rate, high fever, sweating, diarrhea, and muscle twitching. Seek emergency care.

Tardive dyskinesia: Involuntary, repetitive body movements (lip smacking, tongue thrusting, etc.) that may develop with long-term antipsychotic use. May be irreversible. Report any unusual body movements to your doctor.

Severe hyperglycemia: Symptoms like excessive thirst, frequent urination, fruity-smelling breath, or confusion may signal diabetic ketoacidosis — seek emergency care.

DRESS (Drug Reaction with Eosinophilia and Systemic Symptoms): A rare but severe hypersensitivity reaction presenting as a widespread rash, fever, swollen lymph nodes, and organ involvement. Discontinue the medication and seek emergency care if suspected.

Discontinuation Effects

Do not stop Fluoxetine/Olanzapine abruptly. The fluoxetine component can cause SSRI discontinuation syndrome — dizziness, nausea, electric-shock sensations ("brain zaps"), irritability, and anxiety. The olanzapine component can produce rebound symptoms if stopped suddenly. Always work with your prescriber to taper off this medication safely.

Managing Side Effects Day-to-Day

Drowsiness: Take your dose in the evening to allow drowsiness to peak during sleep. Avoid alcohol.

Weight gain: Work with your care team on dietary guidance and regular exercise. Regular metabolic monitoring allows early intervention.

Dizziness on standing: Rise slowly from seated or lying positions. Stay hydrated. Hold onto something stable when getting up.

For information about drug interactions that can make side effects worse, see our guide to Fluoxetine/Olanzapine drug interactions.