Rapamycin (Sirolimus) and Progesterone HRT Interaction: What Patients and Clinicians Need to Know
At a glance
- Drug class / sirolimus is an mTOR inhibitor, FDA-approved (Rapamune) for organ transplant rejection prophylaxis; use for longevity or anti-aging is off-label
- Interaction type / pharmacokinetic (shared CYP3A4/P-gp pathway) plus pharmacodynamic (additive CNS/sedative effect)
- Severity as commonly classified by commercial interaction checkers / moderate, monitor (exact database wording should be re-checked at the time of prescribing, since checker content changes)
- Sirolimus half-life / labeled at roughly 57 to 63 hours in stable patients, meaning a new steady state after a dose or interacting-drug change takes about 5 to 6 days
- Progesterone formulation matters / oral micronized progesterone produces more allopregnanolone and more systemic exposure than vaginal or topical progesterone
- Direct trial evidence quantifying the sirolimus-progesterone trough interaction / not identified in the sources reviewed for this page
- Patient action / report new or worsening drowsiness, confusion, or unsteadiness to the prescriber, and avoid driving in the hours after an oral progesterone dose
The direct answer
Sirolimus and progesterone HRT do not have a described absolute contraindication, and no controlled trial specific to this pairing was located for this review. The interaction concern rests on mechanism: both drugs pass through CYP3A4, sirolimus has essentially no safety margin for unplanned exposure increases, and oral progesterone independently causes sedation through a GABA-A active metabolite that could add to sirolimus's own central nervous system effects at high trough levels. Because the combination has plausible but unquantified pharmacokinetic overlap, the practical approach is a baseline sirolimus trough before starting progesterone, a follow-up trough after the new steady state is reached, and patient counseling on sedation, rather than avoidance of either drug.
What each drug is, so it isn't confused with a relative
Sirolimus (generic name; brand name Rapamune) is an oral mTOR inhibitor FDA-approved for prophylaxis of organ rejection in kidney transplant recipients. It is chemically related to but distinct from other mTOR-pathway drugs such as everolimus and temsirolimus, which have different metabolism profiles and are not addressed here. Sirolimus is also used off-label at lower, often once-weekly doses for longevity or anti-aging purposes; this use has not been evaluated by the FDA and long-term safety data in healthy, non-transplant adults are limited.
Progesterone in hormone replacement therapy can be delivered as oral micronized progesterone (for example, capsules taken at bedtime), vaginal gel or inserts, or compounded topical preparations. These formulations reach very different systemic concentrations, which matters directly for this interaction and is discussed below.
How sirolimus is metabolized
Sirolimus's FDA-approved prescribing information describes it as extensively metabolized by CYP3A4 in the gut wall and liver and as a substrate of the intestinal efflux pump P-glycoprotein. Oral bioavailability is low, and the drug's disposition is sensitive to anything that meaningfully inhibits either pathway. The labeled elimination half-life of roughly 57 to 63 hours in stable patients means a change in a co-administered drug will not show its full effect on trough levels for about 5 to 6 days, which is the basis for timing trough rechecks after any medication change (as described in the FDA-approved Rapamune prescribing information; readers and clinicians should confirm they are viewing the current label revision).
Sirolimus's therapeutic index is narrow. In transplant medicine, target trough ranges are typically in the mid-single to low-teens ng/mL depending on the regimen and institution; off-label longevity dosing (often once weekly) generally targets lower troughs. A modest percentage increase in exposure that is inconsequential for a patient with a low baseline trough could push a patient already near the top of their target range into a level associated with nephrotoxicity, dyslipidemia, or increased infection risk. The exact percentage change attributable to any single interacting drug varies by individual CYP3A4 activity, and general population averages should not be applied to an individual patient's dosing decision.
How progesterone relates to the same pathway
Progesterone is metabolized in the liver, with CYP3A4 contributing to its clearance. As a general pharmacologic principle, a substrate that is present at high enough concentration can competitively occupy the same enzyme used by another drug, slowing that drug's clearance. Whether progesterone at typical HRT doses reaches concentrations high enough to meaningfully compete with sirolimus for CYP3A4 is not established from the material reviewed for this page; in vitro inhibition constants reported in the pharmacology literature for progesterone at CYP3A4 are generally well above the plasma concentrations produced by standard HRT dosing, which argues for a modest rather than large effect in most patients, but this has not been directly measured in sirolimus-treated patients.
Why the route of progesterone changes the risk profile
Oral micronized progesterone undergoes substantial first-pass hepatic metabolism, generating the neurosteroid allopregnanolone, which is a positive allosteric modulator of GABA-A receptors and produces measurable sedation and psychomotor slowing in some users, an effect mechanistically similar to benzodiazepines though generally milder. Vaginal progesterone (gel or inserts) and compounded topical progesterone reach the systemic circulation with far less first-pass hepatic exposure, producing lower systemic progesterone concentrations and correspondingly less allopregnanolone generation. This route difference is well described in the reproductive endocrinology literature as a general pharmacologic property of progesterone delivery, independent of any sirolimus-specific study, and it is the main lever available if a patient on sirolimus needs endometrial protection but wants to minimize sedation and CYP3A4 substrate load.
The additive sedation concern
Sirolimus itself is not classically described as a sedating drug, but its labeled central nervous system adverse effects include headache, insomnia, and tremor, and case reports describe confusion or cognitive slowing at supratherapeutic trough levels. Oral progesterone's allopregnanolone-driven sedation is dose-related and typically most pronounced in the hours after an evening dose. The concern with combining the two is pharmacodynamic addition at the margins: a patient whose sirolimus trough drifts upward because of reduced clearance, combined with oral progesterone's own sedative effect, could experience more drowsiness, unsteadiness, or cognitive slowing than either drug alone would predict. This reasoning is mechanistically sound but has not been tested as a combined outcome in a published trial identified for this review.
Evidence-status interaction assessment
| Status | What this means here | Specific items |
|---|---|---|
| Established | Supported by the FDA label or well-documented general pharmacology | Sirolimus is a CYP3A4/P-gp substrate with a narrow therapeutic index and roughly 57 to 63 hour half-life. Sirolimus is contraindicated in pregnancy per its label. Oral micronized progesterone produces allopregnanolone and measurable sedation; vaginal and topical progesterone produce substantially lower systemic progesterone exposure than oral dosing. |
| Pharmacologically plausible, not directly tested | A reasonable mechanistic hypothesis without a dedicated study confirming magnitude | Progesterone competing at CYP3A4 to raise sirolimus trough levels. Additive CNS depression when an elevated sirolimus trough coincides with oral progesterone's sedative effect. |
| Not established | No trial or outcome data located to support a specific number or clinical conclusion | The size (percentage) of trough elevation attributable to standard-dose progesterone HRT. Whether the combination changes rejection risk, toxicity rates, or any other hard clinical outcome. Whether the interaction differs meaningfully between transplant-dose and off-label once-weekly longevity dosing beyond what lower average troughs would predict generally. |
| Verify before acting on it | Points a prescriber or pharmacist should confirm directly rather than take from a secondary summary | The current revision and exact wording of the Rapamune label in effect at the time of prescribing. The patient's own current sirolimus trough and target range from their transplant or longevity program. Current output from an institutional drug interaction checker (ratings and wording change over time and were not independently re-verified for this page). Whether a formal pharmacokinetic study of sirolimus plus progesterone exists in the primary literature; none was located during this review. |
A reasonable monitoring approach
Because sirolimus's half-life is long, a single trough drawn immediately after starting progesterone will not reflect the new steady state. A commonly used general principle in sirolimus management, drawn from the drug's labeled pharmacokinetics rather than from an interaction-specific trial, is to obtain a baseline trough before a new interacting medication starts, then recheck roughly 5 to 7 days after the new medication has been at a stable dose, since that approximates the time to steady state. If the trough has moved meaningfully outside the individualized target range, a dose adjustment guided by trough level, not by an estimated interaction magnitude, is the standard practice in transplant pharmacology. A simple proportional adjustment (new dose approximately equal to current dose multiplied by target trough divided by measured trough) is a common starting point that should be confirmed with the transplant or prescribing team rather than self-calculated by a patient.
This monitoring principle is general good practice for any suspected CYP3A4 interaction with sirolimus. It has not been validated specifically for progesterone HRT, and the actual size of any trough change from progesterone in a given patient can only be known by measuring it.
What to tell patients
Patients taking oral micronized progesterone are generally advised to take it at bedtime and to avoid driving or operating machinery in the hours immediately after a dose because of sedation. If a patient is also on sirolimus, that sedation caution still applies, and any new or worsening confusion, dizziness, or unsteadiness should be reported to the prescriber rather than assumed to be routine. Grapefruit and grapefruit juice inhibit intestinal CYP3A4 and are well documented to raise sirolimus exposure; this is unrelated to progesterone but is worth repeating because patients on this combination should already be avoiding it. Azole antifungals, macrolide antibiotics, and certain antiretrovirals are stronger CYP3A4 inhibitors than progesterone and warrant their own trough check if added.
Sirolimus is contraindicated in pregnancy based on animal teratogenicity data in its label, and effective contraception is recommended before, during, and for a period after stopping the drug. Progesterone-only HRT dosing is not a reliable contraceptive, so a patient of childbearing potential on this combination needs a separate, confirmed contraceptive method discussed directly with their prescriber.
Special situations worth flagging
Off-label longevity dosing. Many patients using sirolimus outside of transplant medicine take lower, often once-weekly doses that produce lower average trough levels than daily transplant dosing. This likely reduces, but does not eliminate, the theoretical interaction, and trough monitoring remains reasonable given the narrow therapeutic index, even though outcome data in healthy, non-transplant adults on this off-label regimen combined with progesterone HRT are limited.
Reduced CYP3A4 activity. People vary substantially in CYP3A4 enzyme activity for genetic and other reasons. A patient with lower baseline CYP3A4 activity may show a larger-than-average trough change from any competing substrate, including progesterone. Pharmacogenomic testing is not standard of care for this specific combination but may be informative in a patient with otherwise unexplained trough variability.
Hepatic impairment. Reduced liver function lowers sirolimus clearance independent of any drug interaction. Adding oral progesterone in a patient who already has reduced hepatic clearance of sirolimus increases the importance of trough-based dose confirmation rather than relying on a standard maintenance dose.
Evidence boundary
What is established: sirolimus's dependence on CYP3A4 and P-glycoprotein, its narrow therapeutic index, its long half-life, and its pregnancy contraindication are all supported by its FDA label. Oral micronized progesterone's sedative effect through allopregnanolone and the lower systemic exposure of vaginal and topical progesterone relative to oral dosing are well described in the reproductive endocrinology literature generally.
What is plausible but unproven: that progesterone at HRT doses meaningfully raises sirolimus trough levels through CYP3A4 competition, and that this combination produces clinically additive sedation beyond what either drug causes alone.
What is not established: any specific percentage by which progesterone changes sirolimus exposure, and any outcome data (rejection rates, toxicity rates, or longevity-protocol safety data) specific to this drug combination. Readers should not treat a specific numeric interaction magnitude as confirmed unless their own clinician or pharmacist has verified it against current primary literature or an institutional interaction database at the time of prescribing.