What are the common side effects of fluoxetine (Prozac)?

Common Side Effects of Fluoxetine (Prozac)

Fluoxetine commonly causes nausea, insomnia, anxiety, headache, diarrhea, dry mouth, decreased appetite, sexual dysfunction, dizziness, fatigue, and nervousness, with most side effects emerging within the first few weeks of treatment and typically decreasing in frequency over time. 1, 2

Common Physical Side Effects

• Gastrointestinal effects:

  • Nausea (very common)
  • Diarrhea
  • Dry mouth
  • Decreased appetite/anorexia
  • Weight loss (reported in 1.4% of patients versus 0.5% on placebo) 1

• Neurological effects:

  • Headache
  • Dizziness
  • Tremor
  • Somnolence (drowsiness)
  • Fatigue 2, 1

• Other physical effects:

  • Sweating/diaphoresis
  • Weight changes (typically weight loss)
  • Abnormal bleeding (especially when combined with NSAIDs, aspirin, or anticoagulants) 1

Common Psychological Side Effects

• Sleep and arousal:

  • Insomnia (reported in 12-16% of patients versus 7-9% on placebo)
  • Vivid dreams 2, 1

• Mood and cognition:

  • Anxiety/nervousness (reported in 12-16% of patients versus 7-9% on placebo)
  • Restlessness
  • Irritability 1

Serious Potential Adverse Effects

• Psychiatric concerns:

  • Suicidal thinking and behavior (particularly in young adults under 24)
  • Behavioral activation/agitation (motor or mental restlessness, impulsiveness)
  • Hypomania/mania (rare, reported in 0.7% of patients) 2, 1

• Metabolic/physiologic:

  • Hyponatremia (low sodium levels, more common in elderly)
  • Serotonin syndrome (when combined with other serotonergic medications)
  • Seizures (rare, 0.1% of patients) 2, 1

• Sexual dysfunction:

  • Decreased libido
  • Delayed orgasm or anorgasmia
  • Erectile dysfunction 1

Timing of Side Effects

Most side effects of fluoxetine:

• Emerge within the first few weeks of treatment
• Decrease in frequency over time with continued treatment
• May resolve completely in the majority of patients by 6 months 3

Special Populations

• Elderly patients:

  • Higher risk of hyponatremia
  • Should generally avoid fluoxetine due to higher rates of adverse effects 2

• Pregnant women:

  • Potential risk of persistent pulmonary hypertension of the newborn (PPHN) with late pregnancy exposure
  • FDA revised advisory in 2011 states conflicting findings make the PPHN risk unclear 2

• Children and adolescents:

  • Increased risk of behavioral activation/agitation compared to adults
  • Requires closer monitoring for suicidality
  • May experience unique side effects including thirst, hyperkinesia, epistaxis, and menorrhagia 1

Risk Minimization

• Start with lower doses when possible to minimize initial side effects
• Monitor closely during the first few weeks of treatment and after dose increases
• Avoid abrupt discontinuation to prevent withdrawal symptoms
• Be aware of potential drug interactions, particularly with other serotonergic medications
• Educate patients that most side effects tend to improve over time 2, 4

Fluoxetine has a relatively favorable safety profile compared to older antidepressants like tricyclics, with fewer anticholinergic, hypotensive, and cardiovascular effects, and generally less severe consequences in overdose 5.