Addyi (Flibanserin) During Pregnancy and Lactation: What the Evidence Actually Shows

How Flibanserin Works and Why Pregnancy Changes the Equation

Flibanserin is a postsynaptic 5-HT1A receptor agonist and 5-HT2A receptor antagonist that also shows weak agonism at dopamine D4 receptors. This mixed serotonergic-dopaminergic profile shifts the balance of excitatory and inhibitory neurotransmitters involved in sexual desire. The FDA approved it in August 2015 specifically for acquired, generalized hypoactive sexual desire disorder (HSDD) in premenopausal women, making it the first drug approved for this indication.

Pregnancy introduces physiological shifts that alter nearly every pharmacokinetic parameter. Plasma volume expands 40-50% by the third trimester, hepatic CYP3A4 activity increases, and renal clearance rises. Flibanserin undergoes extensive first-pass hepatic metabolism primarily via CYP3A4 and, to a lesser degree, CYP2C19 [1]. These pregnancy-related enzymatic changes could unpredictably alter drug exposure. The blood-brain barrier also undergoes permeability shifts during gestation, and flibanserin is a CNS-active compound designed to cross it.

The BEGONIA trial (N=1,087), the key Phase III study published in the Journal of Sexual Medicine in 2014, enrolled only non-pregnant premenopausal women with HSDD and required reliable contraception throughout participation [2]. No pregnant subject data exist from any of the registration trials. This gap is not unusual for drugs targeting premenopausal women, but it means clinicians must rely entirely on preclinical animal data and pharmacological reasoning when counseling patients.

FDA Labeling: What the Prescribing Information States

The FDA-approved prescribing information for flibanserin does not assign a legacy pregnancy category (A, B, C, D, or X) because its approval came after the June 2015 implementation of the Pregnancy and Lactation Labeling Rule (PLLR), which replaced letter categories with narrative subsections. The label includes three subsections: Pregnancy, Lactation, and Females and Males of Reproductive Potential [1].

Under the Pregnancy subsection, the label states there are no adequate and well-controlled studies in pregnant women. It notes that oral administration of flibanserin to pregnant rats during organogenesis resulted in increased embryofetal mortality and decreased fetal body weights at doses producing maternal systemic exposures approximately 3 times the exposure at the maximum recommended human dose (MRHD) of 100 mg/day. The Lactation subsection discloses that it is unknown whether flibanserin is excreted in human milk but confirms the presence of flibanserin and metabolites in rat milk [1].

The label explicitly advises considering the developmental and health benefits of breastfeeding alongside the mother's clinical need for flibanserin. No formal recommendation to pump and discard is given because the pharmacokinetic data needed to calculate infant exposure simply do not exist.

Animal Reproductive Toxicity Data: Parsing the Signal

The preclinical reproductive toxicology package for flibanserin included studies in both rats and rabbits, the two standard species required by ICH S5(R3) guidelines for pharmaceutical development.

In the rat embryofetal development study, flibanserin was administered orally during gestation days 6 through 17 at doses of 30, 100, and 300 mg/kg/day. At 300 mg/kg/day (approximately 15 times the MRHD on a mg/m² basis), investigators observed increased post-implantation loss, decreased live litter sizes, and reduced fetal body weights. At 100 mg/kg/day (roughly 5 times the MRHD on a mg/m² basis), reduced fetal weight was still present but embryofetal mortality was not significantly elevated. The no-observed-adverse-effect level (NOAEL) for embryofetal development was 30 mg/kg/day, corresponding to approximately 1.5 times the human exposure at the MRHD [1].

In rabbits, oral administration during gestation days 7 through 19 at 15, 50, and 150 mg/kg/day produced maternal toxicity at the highest dose, including decreased body weight and food consumption. Embryofetal effects at 150 mg/kg/day included decreased fetal body weights. The NOAEL in rabbits was 50 mg/kg/day [1].

A pre/postnatal development study in rats at doses up to 200 mg/kg/day showed decreased pup survival and reduced pup body weight during lactation at doses ≥60 mg/kg/day, approximately 3 times the human exposure. Offspring behavioral endpoints (learning, memory, reproductive performance) were not affected at any dose.

These data carry a common limitation in reproductive toxicology: the adverse effects occurred at multiples of human exposure, and direct translation to clinical risk is uncertain. A 2022 review in Reproductive Toxicology noted that supratherapeutic-dose findings in single-species studies should prompt caution but do not automatically predict human teratogenicity when the NOAEL provides a reasonable margin [3].

Lactation: What Rodent Milk Transfer Means for Breastfeeding Decisions

Flibanserin and its metabolites were detected in rat milk at concentrations roughly equivalent to plasma levels [1]. The drug's moderate lipophilicity (LogP ~2.7) and substantial protein binding (~98%) create competing pharmacokinetic forces regarding milk transfer in humans.

High protein binding generally limits free drug available for diffusion into milk. The relatively long 11-hour terminal half-life, though, means the drug maintains steady-state plasma concentrations with daily dosing, which could sustain low-level milk exposure throughout a 24-hour nursing cycle. The estimated relative infant dose (RID) has not been calculated because no human milk studies have been performed. By convention, an RID below 10% is considered compatible with breastfeeding per Hale's criteria, but without measured concentrations, this threshold cannot be applied [4].

Dr. Thomas Hale, a pharmacologist who has authored extensive work on medications in lactation, has noted that for CNS-active drugs with unknown infant exposure data, "the prudent approach is avoidance until human milk studies are available" [4]. This is consistent with the American Academy of Pediatrics' general framework for evaluating CNS-active medications during breastfeeding.

The neonatal blood-brain barrier is more permeable than the adult BBB, particularly in preterm infants. If flibanserin does transfer into milk at pharmacologically relevant concentrations, the infant CNS exposure could be disproportionately higher relative to the plasma level. Sedation, poor feeding, and irritability are theoretical concerns based on the drug's mechanism, though no case reports exist.

Fertility and Conception Planning: Timing the Washout

Flibanserin's terminal half-life of approximately 11 hours means that after 5 half-lives (roughly 55 hours, or about 2.3 days), over 96% of the drug is eliminated from maternal plasma [1]. From a practical standpoint, a woman planning pregnancy should discontinue flibanserin at least one full menstrual cycle before attempting conception. This conservative buffer accounts for individual pharmacokinetic variability, the potential contribution of active metabolites, and the fact that conception date is rarely known with precision.

The fertility subsection of the FDA label reports no effects on mating or fertility in rats at doses up to 10 times the MRHD [1]. Male rat fertility was similarly unaffected. These findings are reassuring but limited. No human fertility data from prospective studies are available.

For women already using flibanserin who discover an unplanned pregnancy, the clinical response should follow the general pharmacovigilance principle for category-unknown drugs: discontinue immediately and counsel that the known animal NOAEL (1.5x human exposure) provides a margin of safety, but no definitive human safety signal exists. Reporting the exposure to the FDA's MedWatch program is appropriate and contributes to post-marketing surveillance.

Why No Human Pregnancy Studies Exist (and Likely Will Not)

Conducting prospective clinical trials of any investigational drug in pregnant women faces ethical and regulatory barriers that are well documented. The 21st Century Cures Act and the Task Force on Research Specific to Pregnant Women and Lactating Women (PRGLAC) have advocated for better inclusion of pregnant and lactating populations in drug development, but HSDD is not a condition that poses maternal or fetal mortality risk [5].

Under the current regulatory framework, a manufacturer has no obligation to conduct pregnancy studies for a drug indicated in a non-life-threatening condition, particularly when the approved population is premenopausal women who are expected to use contraception. Sprout Pharmaceuticals (now part of Valeant/Bausch Health) has not announced any planned pregnancy registry or lactation pharmacokinetic study for flibanserin.

The consequence: clinicians will likely rely on animal data and pharmacological extrapolation for the foreseeable future. Any human exposure data will come from inadvertent pregnancies reported through MedWatch or from case reports. The Infant Risk Center at Texas Tech University Health Sciences Center maintains a helpline (LactMed database via NIH) where providers can report and query lactation exposures, and this database currently lists flibanserin with a recommendation to use an alternative drug [6].

Peripartum HSDD Management: Alternatives When Flibanserin Is Off the Table

HSDD commonly persists or worsens during the postpartum period due to hormonal shifts (particularly declining estradiol and testosterone), sleep deprivation, and psychosocial stressors. When flibanserin is contraindicated due to pregnancy or breastfeeding, the treatment approach shifts to non-pharmacological interventions and, in select cases, off-label hormonal strategies with better-characterized safety profiles.

Cognitive behavioral therapy (CBT) adapted for sexual dysfunction has Level I evidence from a 2018 randomized controlled trial (N=80) published in the Journal of Sexual Medicine [7]. Mindfulness-based interventions for sexual desire have shown effect sizes comparable to flibanserin's benefit over placebo in the BEGONIA trial [2], without any fetal or infant exposure risk.

Transdermal testosterone, while not FDA-approved for women, is used off-label for postmenopausal HSDD and has a more established lactation profile than flibanserin. The Endocrine Society's 2019 guideline on testosterone therapy in women notes that physiologic-dose testosterone (typically 300 mcg/day transdermally) has minimal systemic exposure and is unlikely to produce supraphysiologic levels in breast milk [8]. This option, however, should only be considered postpartum and with close monitoring of maternal and infant androgen levels.

Bremelanotide (Vyleesi), the other FDA-approved pharmacotherapy for premenopausal HSDD, is an injectable melanocortin-4 receptor agonist administered as needed rather than daily. Its pregnancy and lactation profile is similarly limited. The prescribing information reports embryofetal toxicity in rabbits at 8 times the human exposure and no human lactation data [9]. It offers no clear safety advantage over flibanserin during pregnancy or breastfeeding.

The Alcohol Interaction: Amplified Risk in Peripartum Context

Flibanserin carries a boxed warning for severe hypotension and syncope when combined with alcohol. In clinical interaction studies, co-administration with ethanol (0.4 g/kg, approximately 2 standard drinks) produced symptomatic hypotension requiring medical intervention in 17% of female subjects [1].

While alcohol use during pregnancy and lactation is independently discouraged, the interaction is relevant for the peripartum planning window. A woman discontinuing flibanserin who resumes social alcohol consumption should be aware that the drug's effects on blood pressure may persist for several days after the last dose. Syncope risk is particularly dangerous in late pregnancy, when falls carry obstetric consequences including placental abruption.

The REMS program originally required for flibanserin (which mandated prescriber certification and pharmacy enrollment) was modified by the FDA in 2019 to remove pharmacy and prescriber certification requirements while retaining the patient-prescriber counseling component about alcohol. This counseling should specifically address the periconception timeline for women of reproductive age.

Pharmacogenomics: CYP2C19 Poor Metabolizers and Exposure Risk

Approximately 2-5% of Caucasian and 12-23% of East Asian populations are CYP2C19 poor metabolizers [10]. Because CYP2C19 contributes to flibanserin clearance alongside CYP3A4, poor metabolizers may carry higher drug exposure at standard doses. The FDA label notes that CYP2C19 poor metabolizers had approximately 1.5-fold increased flibanserin exposure compared with extensive metabolizers [1].

This pharmacogenomic variability has direct implications for reproductive safety counseling. A CYP2C19 poor metabolizer taking 100 mg daily could have systemic exposure approaching the range where animal studies showed embryofetal effects. Preconception pharmacogenomic testing is not standard practice, but for women with known CYP2C19 poor-metabolizer status, a longer washout period before conception (e.g., 5-7 days rather than 2-3) is a reasonable clinical adjustment.

Concurrent use of moderate or strong CYP3A4 inhibitors (fluconazole, ketoconazole, clarithromycin, certain HIV protease inhibitors) is contraindicated with flibanserin precisely because of the resulting exposure increases of 2- to 4.5-fold [1]. Women in the periconception period should have their medication list reviewed for CYP3A4 inhibitors before establishing a washout timeline.

Contraception Requirements During Active Flibanserin Therapy

The prescribing information does not mandate a specific contraceptive method during flibanserin use, but the key trials (BEGONIA, DAISY, VIOLET) all required participants to use "reliable contraception" [2]. Given the absence of human reproductive safety data, effective contraception during treatment is a clinical expectation, not merely a trial requirement.

Combined hormonal contraceptives (ethinyl estradiol/progestin) have not shown clinically significant pharmacokinetic interactions with flibanserin in dedicated drug-drug interaction studies [1]. This means standard oral contraceptives, patches, or vaginal rings can be used without dose adjustment. IUDs, implants, and depot medroxyprogesterone are similarly acceptable.

The clinical scenario requiring the most careful counseling is discontinuation of contraception while still on flibanserin. Women should discontinue flibanserin first, allow for washout, and then stop contraception. Reversing this sequence creates a window of unprotected fetal exposure during the periconception period, the very window when organogenesis is most vulnerable to teratogenic insults (approximately gestational weeks 3-8).