Vyleesi and Levothyroxine Interaction: What You Need to Know

What These Two Drugs Are

Bremelanotide, sold as Vyleesi, is a melanocortin receptor agonist given by subcutaneous injection. It is FDA-approved for hypoactive sexual desire disorder (HSDD) in premenopausal women, used as needed before anticipated sexual activity [1]. Levothyroxine is synthetic thyroxine (T4), taken orally, usually once daily, to treat hypothyroidism from causes such as Hashimoto's thyroiditis, thyroidectomy, or radioactive iodine ablation [2]. The two drugs treat unrelated conditions, but because hypothyroidism is common in women and disproportionately affects women in the age range that also uses Vyleesi [3], the two are often taken by the same patient.

No pharmacokinetic interaction between bremelanotide and levothyroxine has been established in either drug's FDA labeling. Bremelanotide is given by injection, so it does not compete with oral levothyroxine for absorption in the way an oral drug would. The labeling for bremelanotide does, however, caution that it can slow gastric emptying and that oral drugs taken close in time to a bremelanotide injection may be absorbed more slowly [1]. Because levothyroxine depends on a fairly narrow absorption window to keep TSH in range, this general caution is worth applying even though levothyroxine is not named specifically in either label.

Pharmacokinetics: Two Very Different Delivery Routes

Bremelanotide is dosed as a 1.75 mg subcutaneous injection. According to its labeling, it is not meaningfully metabolized by cytochrome P450 enzymes, is cleared largely by the kidneys, and has a short half-life measured in hours rather than days [1]. Because it bypasses the gastrointestinal tract, it does not directly compete with oral drugs for intestinal absorption.

Levothyroxine's bioavailability depends heavily on gastric pH, fasting state, and intestinal transit time, and the American Thyroid Association (ATA) recommends taking it on an empty stomach, roughly 30 to 60 minutes before food, to keep absorption consistent [4]. Peak absorption occurs in the jejunum and upper ileum. Anything that slows gastric emptying could, in principle, delay how quickly levothyroxine reaches that absorptive segment, even if bremelanotide itself is fully cleared from the bloodstream within hours.

Why This Is a Timing Concern, Not a Metabolic One

This is not a classic CYP450 interaction. Bremelanotide's labeling does not describe clinically significant inhibition or induction of major CYP isoforms at therapeutic doses [1]. The FDA-required interaction studies for bremelanotide looked at co-administration with naltrexone and with an oral drug used as a probe for absorption effects, and the labeling describes reduced or delayed absorption of the co-administered oral drug in those studies [1]. The exact percentages reported in those studies are specific pharmacokinetic figures that should be confirmed directly against the current label before being used to make a clinical decision; they are not restated here as precise numbers because that level of precision could not be independently verified for this article.

What can be stated with more confidence is the general pattern: any drug that meaningfully slows gastric emptying has the theoretical potential to blunt or delay levothyroxine absorption, and levothyroxine has little margin for that kind of variability before TSH drifts out of range. Guideline groups that write about levothyroxine absorption, including the ATA's 2014 treatment guideline and the joint AACE/ATA guideline on hypothyroidism management, describe a general principle that drugs known to interfere with levothyroxine absorption should be separated from levothyroxine dosing by a period of hours, and that new medications should prompt a conversation about thyroid monitoring [4][5]. Bremelanotide is not named in either guideline, because it postdates some of that literature and is not typically grouped with classic absorption inhibitors like calcium, iron, or proton pump inhibitors. The reasoning is extended here by analogy, not because a published guideline specifically addresses bremelanotide.

An older review of levothyroxine pharmacology in the journal Thyroid makes the broader point that the list of medications and supplements known to interfere with levothyroxine absorption keeps expanding, and that clinicians should ask about new medications at thyroid follow-up visits [6]. That is a reasonable general practice to apply here, even though the review predates bremelanotide's approval and does not discuss it directly.

Evidence-Status Assessment: What Is Known, Plausible, or Unverified

A Practical Dose-Timing Approach

For most patients, the simplest solution avoids the theoretical concern entirely. Levothyroxine is typically taken in the early morning on an empty stomach, and bremelanotide is an as-needed medication typically used in the evening before anticipated sexual activity, with a labeled maximum of one dose per 24 hours and no more than 8 doses per month [1]. That pattern naturally separates the two drugs by ten or more hours.

The scenario that deserves more attention is a patient who takes levothyroxine at bedtime, an alternative schedule that some patients use, and who might then use bremelanotide within a few hours of that dose. In that situation, shifting levothyroxine to the morning on days when bremelanotide will be used, or building in a gap of at least several hours between the two, is a reasonable precaution to discuss with a prescriber or pharmacist. This is a general absorption-timing principle applied to a new drug, not a documented dosing rule specific to this combination, and any change to a levothyroxine schedule should go through the prescribing clinician.

Nausea and What It Means for an Oral Thyroid Medication

Nausea is the most commonly reported adverse effect of bremelanotide. In the phase 3 RECONNECT trials that supported FDA approval, nausea was reported by a substantial share of bremelanotide-treated participants, most often mild to moderate, with a smaller proportion experiencing vomiting or discontinuing treatment because of it [7]. The exact percentages appear in the trial publication and current FDA label and should be checked there directly rather than assumed from memory, since this article is not restating the specific figures with high confidence.

The clinical relevance for a levothyroxine user is straightforward: if nausea leads to vomiting within roughly one to two hours of taking levothyroxine, that dose may be partially or fully lost. This is not expected to be common if the two drugs are separated in time as described above, but it becomes more relevant for a patient who takes levothyroxine close to when they use Vyleesi, or who has a history of significant nausea with bremelanotide.

Separately, untreated or undertreated hypothyroidism is itself associated with reduced sexual desire in women in the endocrinology literature [8], which raises a secondary point: absorption problems that push TSH upward could work against the goal of treating HSDD, independent of any direct drug interaction. The specific magnitude of that association reported in some sources could not be confirmed with a matching citation for this article and is stated here only as a general association, not a precise effect size.

Blood Pressure: A Separate, Usually Minor Consideration

Bremelanotide's labeling describes a transient increase in blood pressure after dosing, and the drug is contraindicated in patients with uncontrolled hypertension or known cardiovascular disease [1]. Levothyroxine does not directly raise blood pressure, but over-replacement, meaning a levothyroxine dose that is higher than a patient needs, can increase heart rate and blood pressure through excess thyroid hormone effects [2]. If a patient's levothyroxine dose is not well matched to their needs and bremelanotide is added on top, the combined cardiovascular effect is a reasonable thing to flag with a prescriber, particularly in anyone with existing blood pressure concerns. This is a general cardiovascular caution rather than a documented interaction between the two specific drugs.

Monitoring If You Are Prescribed Both

For a patient already stable on levothyroxine who is starting bremelanotide, a reasonable general approach, consistent with how guideline groups suggest handling any new medication that could affect levothyroxine absorption, includes:

• Confirming a recent TSH and free T4 are on file before or shortly after starting Vyleesi, if labs are not already current.
• Rechecking TSH roughly 6 to 8 weeks after starting bremelanotide, which mirrors the general interval the ATA recommends after any change that could affect levothyroxine absorption [4].
• Telling the prescriber about new or worsening fatigue, cold intolerance, constipation, weight gain, or cognitive sluggishness, which can be symptoms of rising TSH.
• Reporting recurrent vomiting that occurs within a couple of hours of a levothyroxine dose.
• If bremelanotide is stopped after a period of regular use, rechecking TSH again in a similar timeframe, since absorption conditions are changing again.

Any dose adjustment to levothyroxine belongs with the prescribing clinician and should be based on follow-up labs rather than a fixed formula. A single missed or partially absorbed levothyroxine dose is not expected to be clinically significant given the drug's roughly week-long half-life [2]; the concern is a pattern of repeated absorption disruption over weeks.

Other Documented Vyleesi Interactions

The FDA label for bremelanotide describes a few other interaction considerations worth knowing about [1]:

• Naltrexone: co-administration is described as reducing bremelanotide's systemic exposure; concurrent use is generally not recommended. The exact magnitude reported in the label should be confirmed directly rather than assumed.
• Oral drugs with a narrow therapeutic index or a need for rapid onset: the label's general caution about delayed absorption applies broadly to this category, which can include drugs like warfarin or phenytoin; anyone taking such a drug should discuss timing with their prescriber.
• Alcohol: no formal interaction study is described in the label. Because bremelanotide can transiently raise blood pressure and later effects may differ, combining it with alcohol has not been specifically studied and caution is reasonable.
• Hormonal contraceptives: the label does not describe a significant interaction, which is relevant since many patients using Vyleesi for HSDD are also using contraception.

When to Contact a Prescriber

Anyone taking both drugs should contact their prescriber for new or worsening hypothyroid symptoms after starting Vyleesi, for recurrent vomiting that could interfere with levothyroxine dosing, or for any concern about blood pressure if they have existing cardiovascular risk factors. This article does not provide individualized dosing advice; levothyroxine dose changes and the decision to start or continue bremelanotide in a specific patient require an individualized clinical assessment.