Compounded rapamycin delivers less drug per milligram than the commercial pill

Anyone following the longevity literature has seen rapamycin discussed in milligrams: 5 mg a week, 7 mg a week, 10 mg every fortnight. The numbers get quoted between studies, between forums and between clinicians as though they described the same thing.

They do not. A study published in GeroScience in January 2025 examined how much rapamycin actually reaches the blood from different formulations, and found that the milligram figure on a compounded prescription is not comparable to the same figure on a commercial one [s1].

What was measured

The investigators looked at two real-world groups. In a small trial cohort, they measured blood rapamycin 24 hours after self-administered doses in 44 people taking commercial rapamycin at 2, 3, 6 or 8 mg, and 23 people taking compounded rapamycin at 5, 10 or 15 mg [s1].

The dose-to-blood-level relationship was linear for both formulations — more milligrams, more drug in the blood, predictably. But compounded rapamycin produced a lower blood level per milligram than the commercial product [s1]. They then extended the analysis to 316 real-world rapamycin users drawn from an observational database and found similar results, with blood levels peaking about two days after a dose and declining gradually afterwards [s1].

The study also reports two things worth separating. Between different people taking the same dose, blood levels varied considerably. Within the same person, repeat tests over time were relatively consistent [s1]. In other words, the unpredictability is mostly between individuals rather than week to week in one individual.

The authors' own conclusion is that individualised dosing and routine monitoring of blood rapamycin levels should be used [s1]. That is a recommendation from a research group whose members are employees and shareholders of the telehealth company that supplied the cohorts, a conflict the paper discloses [s1].

Why this reframes the trial everyone cites

The PEARL trial is the longest study of low-dose rapamycin for healthy ageing to date, and it used compounded rapamycin, chosen for practical reasons connected to producing a matching placebo [s2].

Midway through the trial the investigators learned about the bioavailability difference, paused, and checked it. Compounded rapamycin turned out to have approximately one-third the blood concentration at 24 hours relative to commercial [s2]. They report that although doses are listed at the advertised compounded amount, the equivalent effective doses are roughly 66% lower — the trial's 10 mg arm corresponding to about 3.33 mg of generic sirolimus [s2].

This matters in both directions, and people tend to notice only one of them.

The reassuring result — adverse events essentially identical to placebo across 48 weeks — was produced at a lower systemic exposure than the labelled dose suggests [s2]. Anyone treating PEARL as a safety clearance for "10 mg weekly" of a commercial product is extrapolating beyond what was tested.

The null result cuts the other way. The trial's primary endpoint, visceral fat, did not move [s2]. If the effective exposure was a third of what the numbers imply, one available explanation is simply that the dose was too low — which is an argument for another trial, not for a conclusion.

The number that is missing

Underneath all of this is an absence that no formulation comparison can fix. There is no established dose-response curve for rapamycin and healthspan extension, despite once-weekly 5 to 7 mg or biweekly 10 to 15 mg being commonly recommended to patients [s3]. The drug is estimated to have biological effects at around 5 ng/mL in the blood and greater relative toxicity above 15 ng/mL [s3], but those thresholds come from its use as an immunosuppressant, not from any trial showing that a particular blood level slows ageing.

So the chain runs: an unvalidated target blood level, reached by an uncertain amount of drug, from a formulation whose potency per milligram depends on where it was made, in pursuit of an effect that has not been demonstrated in humans.

What a reader can take from this

Not a dose. The point of this article is the opposite: the milligram figures circulating in longevity discussion carry less information than they appear to, because they describe the label rather than the exposure, and because no one has established what exposure would even be the right target.

It is also a reminder that a formulation is part of a dose. The comparison found the two routes delivering measurably different amounts of drug at the same labelled milligrams [s1], and the trial that most people cite for safety was run on the weaker of the two [s2]. Any figure quoted without saying which formulation produced it is missing information that the data show to be material.

Rapamycin is a prescription immunosuppressant, prescribed off-label for ageing. Nothing in this article is medical advice, and no dose, formulation or monitoring regimen described here should be read as guidance.

Sources

1. [s1] The bioavailability and blood levels of low-dose rapamycin for longevity in real-world cohorts of normative aging individuals. GeroScience, published online January 28, 2025. https://doi.org/10.1007/s11357-025-01532-w
2. [s2] Influence of rapamycin on safety and healthspan metrics after one year: PEARL trial results. Aging (Albany NY), published online April 4, 2025. https://doi.org/10.18632/aging.206235
3. [s3] What is the clinical evidence to support off-label rapamycin therapy in healthy adults? Aging (Albany NY), published online August 7, 2025. https://doi.org/10.18632/aging.206300