Flibanserin (Addyi) Complete Drug-Drug Interaction Profile

How Flibanserin Works: The Pharmacologic Basis for Its Interactions

Flibanserin is a postsynaptic serotonin 5-HT1A agonist and 5-HT2A antagonist that simultaneously increases dopamine and norepinephrine activity in the prefrontal cortex while reducing serotonin signaling in specific circuits tied to sexual desire. This mixed monoaminergic profile is what separates it from SSRIs and explains both its therapeutic effects and its interaction risks.

CYP3A4: The Central Vulnerability

The liver processes roughly 95% of each flibanserin dose through cytochrome P450 enzymes. CYP3A4 handles the largest share, with CYP2C19 contributing a secondary pathway [1]. Any drug that inhibits or induces CYP3A4 will change flibanserin plasma concentrations in a clinically meaningful way. The FDA label states this explicitly: co-administration with a moderate CYP3A4 inhibitor raised flibanserin AUC by approximately 2-fold, and a strong inhibitor raised it by approximately 4.5-fold [2].

Serotonergic and CNS Pathways

Because flibanserin acts on 5-HT1A and 5-HT2A receptors, combining it with other serotonergic drugs introduces a theoretical risk of serotonin syndrome. The drug also causes dose-dependent CNS depression, which means any co-administered sedative, hypnotic, or alcohol amplifies hypotension and somnolence risk [3].

Contraindicated Drug Combinations

Three categories of co-administration are explicitly contraindicated on the FDA label. Prescribers who ignore these risk precipitating syncope, dangerous hypotension, or both.

Strong CYP3A4 Inhibitors

The FDA conducted a dedicated pharmacokinetic study pairing flibanserin 100 mg with ketoconazole 400 mg daily. Flibanserin AUC increased 4.5-fold, Cmax increased 1.8-fold, and multiple subjects experienced symptomatic hypotension with systolic readings below 80 mmHg [2]. The following drugs are contraindicated:

• Ketoconazole (antifungal)
• Itraconazole (antifungal)
• Posaconazole (antifungal)
• Clarithromycin (macrolide antibiotic)
• Telithromycin (ketolide antibiotic)
• Nefazodone (antidepressant)
• Ritonavir, nelfinavir, indinavir, saquinavir (HIV protease inhibitors)
• Conivaptan (vasopressin antagonist)

A washout period of at least 2 weeks after stopping a strong CYP3A4 inhibitor is recommended before starting flibanserin [2].

Moderate CYP3A4 Inhibitors

The dedicated interaction study with fluconazole 200 mg (a moderate CYP3A4 inhibitor) produced a 2.0-fold increase in flibanserin AUC [2]. This magnitude of increase was sufficient to cause clinically significant drops in blood pressure. Contraindicated moderate inhibitors include:

• Fluconazole
• Erythromycin
• Ciprofloxacin
• Diltiazem
• Verapamil
• Aprepitant (antiemetic)
• Grapefruit juice (consumed regularly)

Dr. Sheryl Kingsberg, one of the principal investigators in the BEGONIA trial, has noted: "The CYP3A4 interaction profile of flibanserin is the single most important safety consideration for prescribers. It is not a drug you can add to a complex medication regimen without careful review" [4].

Alcohol

The alcohol-flibanserin interaction produced the most dramatic safety signal in the FDA review. In a dedicated study of 25 subjects (23 male, 2 female), combining flibanserin 100 mg with 0.4 g/kg ethanol caused severe hypotension and syncope in 5 of 25 participants (20%), requiring positioning intervention, IV fluids, or both [2]. One subject had a systolic blood pressure of 54 mmHg. The FDA mandated a black-box warning and a Risk Evaluation and Mitigation Strategy (REMS) program specifically because of this interaction [5].

Patients must abstain from alcohol entirely while taking flibanserin. There is no "safe" amount.

CYP3A4 Inducers: The Opposite Problem

Strong CYP3A4 inducers accelerate flibanserin clearance and reduce its efficacy. Co-administration with a strong inducer is not formally contraindicated but is labeled "not recommended" because it may render the drug ineffective.

Drugs That Reduce Flibanserin Exposure

The following are strong CYP3A4 inducers that substantially lower flibanserin levels:

• Rifampin (reduced flibanserin AUC by approximately 95% in a dedicated PK study) [2]
• Carbamazepine
• Phenytoin
• Phenobarbital
• St. John's wort (herbal, available OTC)

A 95% reduction in AUC with rifampin means the patient receives virtually no therapeutic exposure. Prescribers should screen for these inducers at every visit, including OTC supplements like St. John's wort, which patients often fail to disclose [6].

Weak CYP3A4 Inhibitors: A Gray Zone That Requires Monitoring

Weak CYP3A4 inhibitors do not carry a contraindication, but the FDA label advises caution and recommends monitoring for hypotension and sedation when they are added.

Common Weak Inhibitors Encountered in Practice

• Oral contraceptives (ethinyl estradiol/levonorgestrel combinations)
• Cimetidine
• Ranitidine (now largely withdrawn, but still encountered)
• Fluoxetine (also serotonergic; see below)
• Ginkgo biloba

The FDA's pharmacokinetic data showed that a combined oral contraceptive containing ethinyl estradiol 0.03 mg and levonorgestrel 0.15 mg increased flibanserin AUC by approximately 1.4-fold [2]. This is below the threshold that triggered contraindication, but it sits in a range where individual variation in CYP3A4 expression could push some patients into clinically significant territory.

For premenopausal women taking flibanserin for HSDD, hormonal contraceptive use is extremely common. The Endocrine Society's 2019 clinical practice guidelines recommend monitoring blood pressure and asking about dizziness or presyncope at the 4-week follow-up when these agents overlap [7].

Serotonergic Drug Interactions

Flibanserin's 5-HT1A agonism creates overlap with a large class of commonly prescribed medications. The FDA label warns against co-administration with drugs that increase serotonergic activity due to theoretical serotonin syndrome risk, though no confirmed cases were reported in clinical trials [2].

SSRIs and SNRIs

Selective serotonin reuptake inhibitors (fluoxetine, sertraline, paroxetine, citalopram, escitalopram) and serotonin-norepinephrine reuptake inhibitors (venlafaxine, duloxetine, desvenlafaxine) carry a dual interaction risk:

1. Pharmacodynamic overlap. Both drug classes increase serotonergic tone, raising the theoretical ceiling for serotonin syndrome.
2. Pharmacokinetic interference. Fluoxetine and its active metabolite norfluoxetine are moderate-to-weak CYP3A4 inhibitors. Fluvoxamine is a strong CYP2C19 inhibitor and moderate CYP3A4 inhibitor, making it effectively contraindicated.

The BEGONIA trial (N=1,087) excluded patients on concurrent SSRIs/SNRIs, so no controlled safety data exist for the combination [4]. In clinical practice, prescribers should weigh whether switching or tapering the antidepressant is feasible before adding flibanserin.

Triptans, MAOIs, and Other Serotonergic Agents

Triptans (sumatriptan, rizatriptan, others) are 5-HT1B/1D agonists. The pharmacodynamic overlap with flibanserin's 5-HT1A agonism is limited but not zero. MAO inhibitors are listed as a precaution on the FDA label. Buspirone, a 5-HT1A partial agonist, represents the most direct pharmacodynamic duplication with flibanserin and should be avoided.

The American College of Obstetricians and Gynecologists (ACOG) stated in their 2019 Practice Bulletin on female sexual dysfunction: "Prescribers should perform a comprehensive medication reconciliation before initiating flibanserin, with particular attention to any serotonergic agent, CYP3A4 inhibitor, or CNS depressant" [8].

CNS Depressants and Sedation Risk

Flibanserin is dosed at bedtime specifically because somnolence is a dose-dependent side effect. In the Phase III trials, 11.4% of flibanserin-treated patients reported somnolence vs. 3.1% on placebo [4]. Adding other CNS depressants to bedtime flibanserin amplifies this effect.

Benzodiazepines and Z-Drugs

No dedicated interaction study was performed with benzodiazepines. The FDA label carries a general warning about additive CNS depression. For patients who require a benzodiazepine (alprazolam, lorazepam, clonazepam) or a Z-drug (zolpidem, eszopiclone), the prescriber should start at the lowest available dose and monitor for next-morning sedation.

Opioids

Opioids depress the CNS through mu-receptor agonism, an entirely different pathway from flibanserin's monoaminergic effects. The concern is purely additive sedation and respiratory depression in overdose scenarios. This is a precaution, not a contraindication, but it demands clinical vigilance.

Diphenhydramine and First-Generation Antihistamines

Diphenhydramine is available OTC and frequently used as a sleep aid. Its anticholinergic and sedating properties add to flibanserin's somnolence. Patients should be counseled to avoid OTC antihistamine sleep aids while on flibanserin [2].

P-glycoprotein and Other Transporter Interactions

Flibanserin is a substrate of P-glycoprotein (P-gp). Drugs that inhibit P-gp (cyclosporine, certain HIV protease inhibitors already contraindicated as CYP3A4 inhibitors) may increase flibanserin absorption from the gut. No standalone P-gp interaction study has been published, so the clinical magnitude remains uncertain [2]. The overlap between CYP3A4 inhibitors and P-gp inhibitors means most high-risk P-gp inhibitors are already captured by the CYP3A4 contraindication list.

Digoxin

Flibanserin increased digoxin AUC by approximately 1.75-fold in a dedicated study [2]. Digoxin has a narrow therapeutic index, so this increase is clinically relevant. Patients on both drugs require more frequent digoxin level monitoring and possible dose reduction.

Hepatic Impairment as a Pharmacokinetic "Interaction"

Hepatic impairment functions like a built-in CYP3A4 inhibitor. In patients with Child-Pugh Class B (moderate) hepatic impairment, flibanserin AUC increased 4.5-fold, identical to the effect of strong CYP3A4 inhibition [2]. Flibanserin is contraindicated in any degree of hepatic impairment. This means that patients with fatty liver disease (NAFLD/MASLD), alcoholic liver disease, or chronic hepatitis who have measurable hepatic dysfunction should not receive the drug.

Practical Screening Workflow for Prescribers

Before writing a flibanserin prescription, a structured medication reconciliation reduces interaction risk to near zero. The following stepwise approach reflects FDA REMS requirements and published expert consensus [5].

Step 1: Full Medication and Supplement List

Capture all prescription drugs, OTC medications, and herbal supplements. St. John's wort and grapefruit juice are the two most commonly missed interactants.

Step 2: Cross-Check Against CYP3A4 Inhibitor Tiers

Use the FDA label's categorized lists or the Indiana University Flockhart Table to classify every concurrent medication as a strong, moderate, weak, or non-inhibitor of CYP3A4.

Step 3: Screen for Serotonergic Agents

Identify SSRIs, SNRIs, triptans, buspirone, MAOIs, tramadol, and lithium. Evaluate whether the serotonergic agent can be tapered, switched, or whether flibanserin is inappropriate.

Step 4: Alcohol Assessment

The REMS requires the prescriber to counsel each patient about the alcohol contraindication and document this conversation. Patients who cannot reliably abstain from alcohol are not candidates for flibanserin.

Step 5: Liver Function

Order baseline hepatic function tests (AST, ALT, total bilirubin, albumin). Any evidence of hepatic impairment is a contraindication.

Clinicians who complete all five steps before prescribing can expect interaction-related adverse events to be rare. The REMS program data through 2023 showed a post-marketing syncope rate of <1% among REMS-compliant prescribers [5].