Duloxetine

Mechanism

Ciprofloxacin is a potent CYP1A2 inhibitor, the main duloxetine metabolic route. Duloxetine plasma levels rise 6-fold. Nausea, sedation, and serotonergic effects emerge.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, hypertension, fever. First signs appear within hours of co-administration.

Management

The AEMPS label for duloxetine lists the combination with ciprofloxacin as contraindicated: ciprofloxacin, a strong CYP1A2 inhibitor, sharply raises duloxetine exposure. Alternative antibiotics without CYP1A2 effect: a cephalosporin, nitrofurantoin, or fosfomycin.

Mechanism

Dual risk. Fluvoxamine is a potent CYP1A2 inhibitor, the main duloxetine metabolic route. Duloxetine plasma levels rise 6-fold. Plus additive serotonergic load.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, hypertension, fever. First signs appear within hours of co-administration.

Management

The AEMPS label for duloxetine lists the combination with fluvoxamine as contraindicated. The combination is not prescribed. Choose one antidepressant: duloxetine for pain syndromes, fluvoxamine for obsessive-compulsive disorder. For augmentation, add mirtazapine.

Mechanism

Linezolid is a reversible monoamine oxidase (MAO) inhibitor. Duloxetine (SNRI) raises synaptic serotonin and norepinephrine. Co-administration: serotonin syndrome (FDA safety alert 2011, fatal cases reported).

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, hypertension, fever. First signs appear within hours of co-administration.

Management

The AEMPS label for duloxetine says linezolid must not be given to patients on duloxetine. The combination is not prescribed. Stop duloxetine at least 5 days before starting linezolid. For urgent antibiotic therapy: vancomycin, daptomycin, or ceftaroline by susceptibility.

Mechanism

Additive seizure threshold reduction plus elevated synaptic norepinephrine (bupropion is a norepinephrine-dopamine reuptake inhibitor; duloxetine is an SNRI). Tachycardia and hypertension may emerge.

Symptoms

Tachycardia, hypertension, tremor, agitation. Seizures may occur, especially with prior epilepsy or eating disorders.

Management

The combination is possible for antidepressant augmentation with careful blood pressure and pulse monitoring. Cap bupropion at 300 mg and duloxetine at 60 mg/day. Avoid with prior epilepsy or eating disorders.

Mechanism

SSRI + SNRI – serotonergic load rises, serotonin syndrome risk. Duloxetine also prolongs QT at high doses, summing with citalopram's QT effect.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, hypertension, fever. First signs appear within hours of co-administration.

Management

The combination is not prescribed. When switching antidepressants, allow a 2-week washout. Alternative monotherapy: mirtazapine or agomelatine.

Mechanism

SSRI + SNRI – serotonergic load rises, serotonin syndrome risk. The combination has no therapeutic rationale.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, hypertension, fever. First signs appear within hours of co-administration.

Management

The combination is not prescribed. When switching antidepressants, allow a 1-week washout. For augmentation, use mirtazapine or agomelatine.

Mechanism

Fluoxetine is a potent CYP2D6 inhibitor; duloxetine is minorly metabolised by CYP2D6. Additive serotonergic load with dual SSRI/SNRI dosing.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, hypertension, fever. First signs appear within hours of co-administration.

Management

The combination is not prescribed. When switching from fluoxetine to duloxetine, allow a 5-week washout (due to norfluoxetine); from duloxetine to fluoxetine, 1 week.

Mechanism

Mirtazapine + SNRI – augmentation in resistant depression ('California rocket fuel'). Serotonin syndrome is rare but possible at high doses.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, hypertension, fever. First signs appear within hours of co-administration.

Management

The combination is possible for augmentation. Keep doses low: mirtazapine 15–30 mg, duloxetine 30–60 mg/day. Monitor blood pressure, pulse, and mental status in the early weeks.

Mechanism

SSRI (sertraline) + SNRI (duloxetine) – additive serotonergic load, serotonin syndrome risk without therapeutic rationale.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, hypertension, fever. First signs appear within hours of co-administration.

Management

The combination is not prescribed. When switching antidepressants, allow a 1-week washout. For augmentation, use mirtazapine or agomelatine.

Mechanism

Dual SNRI – doubling the same pharmacological effect without therapeutic benefit. Risk of serotonin syndrome, hypertension, and tachycardia.

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, hypertension, fever. First signs appear within hours of co-administration.

Management

The combination is not prescribed. Choose one: venlafaxine for depression or generalised anxiety disorder, duloxetine for pain syndromes (diabetic neuropathy, fibromyalgia). When switching, allow a 1-week washout.

Mechanism

Duloxetine is a serotonin-norepinephrine reuptake inhibitor (SNRI). It depletes the platelet serotonin pool, impairing primary haemostasis. On warfarin, upper GI bleeding risk roughly doubles.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

Management

The combination is acceptable with coagulation monitoring. Check the INR more often than usual after duloxetine is started or stopped, until values are stable. Tell the patient to report bruising without injury, bleeding gums, black stools or blood in the urine. Bleeding risk rises further if NSAIDs or aspirin are added, and the duloxetine label calls for caution in that case.

Mechanism

Duloxetine inhibits the reuptake of serotonin and noradrenaline. Platelets release serotonin when they stop bleeding, so such antidepressants raise the risk of bleeding, including from the upper gastrointestinal tract. Aspirin irreversibly blocks platelet cyclooxygenase and injures the stomach lining. According to the US Food and Drug Administration (FDA) duloxetine label, aspirin may add to the bleeding risk during duloxetine treatment.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, black or tarry stools. Severe cases: GI bleeding.

Management

The combination can be used with caution: the Spanish and EU duloxetine SmPCs ask for caution in patients taking drugs that affect platelet function and name aspirin. The US Food and Drug Administration (FDA) requires prescribers to warn patients about the higher bleeding risk when duloxetine is combined with aspirin (duloxetine FDA label). Explain the signs of bleeding to the patient in advance: bruising without injury, nosebleeds, black stools, vomiting blood. With black stools, vomiting blood or any other sign of gastric bleeding, the patient stops aspirin and sees a doctor at once (aspirin 100 mg SmPC).

Mechanism

Additive CNS depression. Duloxetine weakly inhibits CYP2D6 with no CYP3A4 effect — no pharmacokinetic interaction with alprazolam.

Symptoms

Sedation, fatigue, slowed reactions, impaired coordination. In older patients: fall and fracture risk. Driving and operating machinery: accident risk.

Management

On starting the combination, begin alprazolam at the minimum dose. In anxiety with chronic pain (neuropathy, fibromyalgia), the combination is reasonable, but for long-term therapy assess feasibility of benzodiazepine discontinuation via CBT. Avoid the combination in older patients — fall risk.

Mechanism

Duloxetine is cleared by CYP1A2 and CYP2D6 (duloxetine label, US Food and Drug Administration, FDA). Amiodarone and its metabolite desethylamiodarone inhibit both enzymes (amiodarone SmPC), so duloxetine levels may rise. Amiodarone is not named as a potent CYP1A2 inhibitor: the duloxetine contraindication covers only fluvoxamine, ciprofloxacin and enoxacin (duloxetine SmPC). Duloxetine does not prolong the QT interval, even at 2.7–3.3 times the maximum dose (duloxetine label, FDA).

Symptoms

The combination may intensify the usual duloxetine adverse effects: nausea, headache, dry mouth, drowsiness and dizziness (duloxetine SmPC). Duloxetine can raise blood pressure, up to hypertensive crisis in patients with pre-existing hypertension (duloxetine SmPC). Duloxetine overdose has caused drowsiness, seizures, vomiting, tachycardia, serotonin syndrome and coma (duloxetine SmPC).

Management

The combination is acceptable with monitoring: the duloxetine SmPC asks for caution when it is given with drugs that impair its metabolism (duloxetine SmPC). In patients with heart disease, blood pressure is monitored, especially in the first month of duloxetine treatment, and a sustained rise calls for a lower duloxetine dose or gradual withdrawal (duloxetine SmPC). Amiodarone interactions can persist for several months after it is stopped (amiodarone SmPC).

Mechanism

The combination works along two lines. Kinetics: duloxetine is a moderate inhibitor of CYP2D6, an enzyme that clears amitriptyline (labels of both drugs). With another tricyclic, desipramine, duloxetine tripled AUC (duloxetine SmPC). Serotonin: both drugs block the reuptake of serotonin and noradrenaline, and the combination raises the risk of serotonin syndrome (labels of both drugs).

Symptoms

Serotonin syndrome: agitation, hallucinations, coma, tachycardia, labile blood pressure, fever, hyperreflexia, incoordination, nausea, vomiting, diarrhoea. In its most severe form it resembles neuroleptic malignant syndrome, with fever and muscle rigidity (duloxetine SmPC). Signs of amitriptyline excess: drowsiness, dry mouth, constipation, urinary retention, fast heartbeat, dizziness on standing, and at high levels arrhythmias and QT prolongation (amitriptyline SmPC).

Management

Use the combination with caution (labels of both drugs). Watch for serotonin syndrome at the start of treatment and after each dose increase. The FDA labels of both drugs require stopping both drugs at once and starting supportive treatment if it appears. The amitriptyline plasma level may need monitoring and its dose may need reducing (FDA duloxetine label). Measure blood pressure before starting duloxetine and periodically during treatment, and do not start duloxetine in uncontrolled hypertension (duloxetine labels). Use amitriptyline with caution in marked bradycardia, decompensated heart failure and with QT-prolonging drugs; potassium or magnesium disturbances raise the risk of arrhythmia (amitriptyline SmPC).

Mechanism

Duloxetine (SNRI) moderately impairs primary haemostasis by inhibiting platelet serotonin reuptake. Additively with apixaban, GI bleeding risk rises.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, black or tarry stools, haematuria. Higher risk in older patients, prior peptic ulcer disease, and chronic kidney disease.

Management

The combination is acceptable. In patients over 65 or with prior peptic ulcer: pantoprazole 20 mg/day. For long-term therapy, alternative antidepressants without platelet effect: mirtazapine, agomelatine.

Mechanism

The metronidazole and duloxetine documents do not mention each other, neither the Spanish SmPCs nor the US Food and Drug Administration (FDA) labels. No kinetic pathway is described: duloxetine is cleared through CYP1A2 and CYP2D6 (duloxetine SmPC) and does not inhibit CYP2C9 or CYP3A (FDA duloxetine label), while the metronidazole SmPC lists interactions with warfarin, lithium, ciclosporin, phenytoin, 5-fluorouracil and busulfan but none with antidepressants. Metronidazole's own clearance is slowed by inhibitors of hepatic microsomal enzymes such as cimetidine (FDA metronidazole label), and duloxetine is not one of them. The documents give metronidazole no serotonergic activity, and the metronidazole warning about QT prolongation with QT-prolonging drugs does not apply to duloxetine: in the duloxetine SmPC approved by the European Medicines Agency (EMA) the rate-corrected QT interval of patients on duloxetine did not differ from placebo, and the FDA duloxetine label found no QT prolongation even at doses above the maximum. The only overlap is the liver: duloxetine has caused severe liver enzyme elevation, hepatitis and jaundice, and its SmPC asks for caution with other medicines associated with liver injury (EMA duloxetine SmPC). The metronidazole SmPC lists enzyme elevation, cholestatic or mixed hepatitis, hepatocellular damage and cases of liver failure among its adverse reactions and describes rapid, sometimes fatal hepatotoxicity in Cockayne syndrome.

Symptoms

The documents describe no symptoms specific to the interaction. Watch for the signs of liver injury that each document lists for its own drug: abdominal pain, enlarged liver, rising transaminases, jaundice (US Food and Drug Administration (FDA) duloxetine label and European Medicines Agency (EMA) duloxetine SmPC), raised liver enzymes and hepatitis with jaundice (metronidazole SmPC, section 4.8). In Cockayne syndrome, liver failure with metronidazole has developed as early as 2 days after the start of treatment (FDA metronidazole label).

Management

By the documents, neither drug needs a dose change because of the other: the metronidazole and duloxetine documents do not mention the pair and describe no kinetic interaction. The only instruction concerns the liver: duloxetine is used with caution in patients taking other medicines associated with liver injury (duloxetine SmPC approved by the European Medicines Agency, EMA), and metronidazole's own SmPC places it among such medicines (section 4.8). So during a combined course, watch for signs of liver injury, and if jaundice or other evidence of clinically significant liver dysfunction appears, stop duloxetine and do not resume it unless another cause is established (US Food and Drug Administration (FDA) duloxetine label). In Cockayne syndrome metronidazole is contraindicated (FDA metronidazole label); the Spanish metronidazole SmPC allows it only when benefit outweighs risk and no alternative exists, with liver function tests before, during and after treatment and withdrawal if values rise markedly. Duloxetine is contraindicated in liver disease causing hepatic impairment (EMA duloxetine SmPC), and that restriction applies regardless of metronidazole. No electrocardiogram is needed for the combination: by its own documents duloxetine does not prolong the QT interval.

Mechanism

Duloxetine is a serotonin and noradrenaline reuptake inhibitor (SNRI). Tramadol also inhibits serotonin and noradrenaline reuptake, so together they add up serotonergic effects and raise the risk of serotonin syndrome. Tramadol lowers the seizure threshold, and SNRIs increase this risk (tramadol SmPC). There is a kinetic part too: duloxetine is a moderate CYP2D6 inhibitor (duloxetine SmPC), and tramadol needs CYP2D6 to form the metabolite that gives most of its analgesia. Less active metabolite is formed and more tramadol itself stays in the blood (FDA tramadol label).

Symptoms

Serotonin syndrome: agitation, confusion, tremor, myoclonus, hyperreflexia. Autonomic features: sweating, tachycardia, labile blood pressure, fever. Gastrointestinal: nausea, vomiting, diarrhoea. Seizures can occur on this combination. When less active tramadol metabolite is formed, pain relief is weaker than expected.

Management

Use the combination when it is clinically warranted (both SmPCs). In a patient with depression, the FDA tramadol label advises considering a non-opioid analgesic first. Keep tramadol at or below 400 mg a day, because seizure risk rises above it. Tramadol is contraindicated in epilepsy that is not controlled; in controlled epilepsy it is used only when the circumstances require it (tramadol SmPC). Watch for signs of serotonin syndrome at the start of co-treatment and after each dose increase. If they appear, stop tramadol and duloxetine at once and start symptomatic treatment (FDA labels of both drugs). Measure blood pressure before starting duloxetine and periodically during treatment, and do not start duloxetine in uncontrolled hypertension (duloxetine labels). If duloxetine is stopped and tramadol continues, more active metabolite is formed: consider lowering the tramadol dose and watch for sedation and slowed breathing (FDA tramadol label).