Duloxetine is a serotonin and norepinephrine reuptake inhibitor (SNRI), first patented in 1990 and marketed in the United States in 2004. It is FDA-approved for use in the treatment of major depressive disorder, generalized anxiety disorder, fibromyalgia, chronic musculoskeletal pain, and diabetic peripheral neuropathy. Off-label (non-FDA approved) use for duloxetine include chemotherapy-induced peripheral neuropathy and stress urinary incontinence in both men and women.[1]

In addition to its general SNRI properties, duloxetine also increases dopamine levels, specifically in the prefrontal cortex. Due to the involvement of the prefrontal cortex in depression, the actions of duloxetine in this region also contribute to its efficacy in the treatment of depression. However, the mechanism of action does not involve the inhibition of dopamine transporters.[2]

Duloxetine inhibits the reuptake of both serotonin and norepinephrine, thus combining two therapeutic mechanisms in one agent to treat depression and anxiety. As well, duloxetine enhances dopamine levels within the prefrontal cortex. The mechanism of action behind the increase in dopamine levels involves the inhibition of norepinephrine transporters. These transporters have a significant affinity for dopamine, resulting in the pump’s ability to act on both dopamine and norepinephrine. Therefore, inhibition of norepinephrine transporters can lead to an increase in dopamine. This increase in dopamine specifically takes place in the prefrontal cortex where dopamine transporters are scarce, and reuptake relies more heavily on norepinephrine transporters.[3]

Duloxetine works to treat various neuropathic and chronic pain states by increasing the activity of noradrenergic and serotonergic neurons in the descending spinal pathway on the dorsal horn. These descending neurons inhibit the activity of dorsal horn neurons, suppressing excessive input from reaching the brain. The hypothesis is that a deficiency in these inhibitory signals results in excess input reaching the brain, which gets perceived as pain.[4]

Of note, duloxetine has no significant activity for muscarinic, cholinergic, alpha2-adrenergic, or H1 histaminergic receptors.

Duloxetine is most commonly administered in a capsule form due to good absorption via the oral route, and administration can be with or without food. Duloxetine should not be crushed or chewed. However, studies have shown that duloxetine may remain stable up to 2 hours after being opened and sprinkled on apple juice or apple sauce.

Management of fibromyalgia: 30mg once daily can be given for one week, then the dose may be increased to 60 mg once daily. A titration approach may also be utilized starting with 20 mg once daily, then increasing the dose by 20 mg every week, up to 60 mg, as tolerated. Maximum dose: 60 mg/day.[5]

Treatment of generalized anxiety disorder (GAD): 60mg can be administered initially, however depending on patient tolerance, 30 mg may be given once daily for one week, then the dose may be increased to 60mg once daily. Though studies have shown no additional benefit to dose above 60 mg/day, some experts have reported benefit when increasing dose over 60 mg/day.

Treatment of unipolar major depressive disorder (MDD): 40 to 60 mg/day can be given in a single or two divided doses. It may be necessary to start at 30mg/day for one week, and then the dose may be increased to 60 mg/day. Maintenance: 60 mg/day. Though studies have shown no additional benefit to dose above 60 mg/day, some experts have reported benefit when increasing dose over 60 mg/day.

Management of pain associated with diabetic peripheral neuropathy: 60 mg can be administered initially, however depending on patient tolerance, lower starting doses may be appropriate. Maximum dose: 60 mg/day.

Management of chronic musculoskeletal pain, including osteoarthritis of the knee and low back pain:

Low back pain: 30 mg can be given once daily for one week and increased up to 60 mg once daily as tolerated as an adjunct therapy. Maximum dose: 60 mg/day.

Knee pain: 30 mg can be given once daily for one week and increased up to 60 mg once daily as tolerated as an adjunct therapy.[6]

Duloxetine has a very low anticholinergic side effect profile; adverse effects related to the cardiovascular, gastrointestinal, central nervous system, such as headache and drowsiness, and fatigue are more common.[1][6]

Serious adverse effects of duloxetine include:

• Suicidality
• Serotonin syndrome
• Hepatoxicity
• Mania
• Syncope
• SIADH
• Hyponatremia

Common adverse effects of duloxetine include:

• Headache
• Drowsiness
• Fatigue
• Nausea
• Xerostomia
• Abdominal pain
• Weight loss
• Weakness
• Insomnia
• Dizziness
• Libido changes
• Diaphoresis
• Constipation
• Decreased appetite
• Tremor
• Diarrhea
• Erectile dysfunction

Duloxetine is contraindicated in patients with concurrent or recent (within two weeks) therapy with a monoamine oxidase (MAO) inhibitors, uncontrolled acute-angle glaucoma, or hypersensitivity to duloxetine. Duloxetine should also be avoided in patients with liver failure or severe renal dysfunction. As well, clinicians should avoid using duloxetine in patients receiving treatment with linezolid or IV methylene blue due to an increased risk of serotonin syndrome.[1]

Use in Special Populations:

Pregnant Patients:

Duloxetine is FDA pregnancy category C, which means risk to fetal development cannot be ruled out. In animal reproductive studies, duloxetine demonstrated adverse effects on embryo/fetal development. Therefore, the clinician should only give duloxetine if the potential benefits justify the potential risk to the fetus.

Breastfeeding Women

Duloxetine is excreted in breast milk and may produce effects in the nursing infant. Some studies show the relative infant dose (RID) of duloxetine to be 2.3%. RID is the dose received via breast milk relative to the mother’s dose. A RID below 10% is acceptable when breastfeeding. Some sources recommend breastfeeding only if the RID is less than 5% if the mother is on psychotropic medication. Regardless, an infant breastfed by a mother receiving duloxetine requires monitoring for behavior changes, sleep disturbance, feeding changes, growth, and neurodevelopment.[7]

Elderly

SSRIs and SNRIs have correlations with several cases of clinically significant hyponatremia in elderly adults. Caution is necessary when prescribing duloxetine in the geriatric population. However, dosage adjustment based on the age of the patient is not required.[8]