Progesterone

Limits on what this shows

• Key negative/null findings: No human trial measuring mitochondrial parameters in progesterone users was located; the effect is not established outside this specific ovariectomized-rat brain model.
• At-risk subgroups: None identified; the direction found is protective, not depleting.

Regulatory status

Not addressed in the FDA label.

Population evidence

Ovariectomized rats treated with progesterone in vivo; isolated whole-brain mitochondria subsequently assessed ex vivo for respiration and electron transport chain complex IV (cytochrome c oxidase) expression/activity using oxygen-electrode assays.

Mechanism

Named, animal (Tier B). Progesterone treatment increased complex IV expression/activity and ADP-coupled respiratory efficiency in brain mitochondria.

Counter-evidence held on record

This finding shares its underlying source with the ATP production and Oxidative stress entries below (see Overlap map) — it is one animal study, not three independent confirmations. Reverse-direction (protective) animal finding only. Must not be marketed as a human mitochondrial-support claim or extrapolated to standard oral/vaginal therapeutic dosing in non-ovariectomized adults.

Limits on what this shows

• Key negative/null findings: No human oxidative-stress-biomarker trial in progesterone users (outside acute neurotrauma research, which has its own separate literature and outcomes) was located during this review.
• At-risk subgroups: None identified; direction is protective in both animal models.

Regulatory status

Not addressed in the FDA label.

Population evidence

(1) Ovariectomized-rat brain-mitochondria study: progesterone treatment decreased reactive-oxygen-species leak and lipid peroxidation. (2) Rat subarachnoid-hemorrhage model, n=72 rats in 4 groups of 18, progesterone 8 or 16 mg/kg given at 1, 6, and 12 hours post-hemorrhage: progesterone "elevated the activity of SOD and decreased MDA content" at 24 hours, with a dose-response relationship.

Mechanism

Named, animal (Tier B) in both. Reduced mitochondrial ROS leak/lipid peroxidation in the uninjured brain-mitochondria model; increased superoxide dismutase activity and decreased malondialdehyde (a lipid-peroxidation marker) in the acute-injury model.

Counter-evidence held on record

The acute-SAH dosing (8–16 mg/kg in rats, given repeatedly post-injury) is a pharmacologic neuroprotection dose in an injury model, not comparable to standard human HRT/luteal-support dosing; it should not be read as evidence of antioxidant benefit at ordinary therapeutic exposure. Reverse-direction (protective) animal finding only, bounded to specific injury/estrogen-deprivation models. Not a basis for a general human antioxidant-support claim.

Limits on what this shows

• Key negative/null findings: The source measured bleeding control as the outcome, not ferritin, hemoglobin, or transferrin saturation; per Hard Rule 2, the bleeding-control outcome itself is not filed as an iron finding, only the plausible downstream implication is noted here as mechanistic.
• At-risk subgroups: Does not generalize beyond the abnormal-uterine-bleeding treatment context; routine luteal-support or cyclic HRT dosing regimens are not expected to meaningfully reduce menstrual iron loss in the same way, since they are not typically used to treat heavy bleeding.

Regulatory status

Not addressed in the FDA label; abnormal uterine bleeding management with progesterone is an established but off-label/adjunct clinical use pattern, not a label indication for Prometrium specifically.

Population evidence

Case report/series, adolescent females with dysfunctional uterine bleeding treated with micronized progesterone (Tier B).

Mechanism

Reduced heavy/abnormal uterine bleeding lowers cumulative menstrual iron loss (standard menstrual-iron physiology); this is an indirect, bleeding-mediated pathway, not a direct progesterone-iron interaction.

Counter-evidence held on record

This is a small case series, not a controlled trial with iron biomarkers as an endpoint; confounding by indication (bleeding severity, underlying cause of the abnormal bleeding) is not excluded. Not a basis for recommending against iron supplementation generally in progesterone users; bounded strictly to the abnormal-uterine-bleeding treatment context, where clinical judgment about bleeding control (not this dossier) should drive iron-related decisions.

Limits on what this shows

• Key negative/null findings: The human arm is a trimester-comparison correlation, confounded by the many other hormonal and physiological changes of pregnancy; it does not isolate progesterone as the sole driver in humans.
• At-risk subgroups: None identified; direction is protective (enrichment of a generally beneficial genus).

Regulatory status

Not addressed in the FDA label.

Population evidence

Human: 16S rRNA sequencing in 35 pregnant women, 1st trimester vs 3rd trimester; Bifidobacterium relative abundance rose significantly (p<0.001). Mouse: subcutaneous progesterone pellets (~1.67 mg/day, 21 days) vs placebo, n=7 vs 9; Bifidobacterium 0.23% vs 0.0007% (p=0.0007). In vitro: direct fecal-culture exposure to progesterone vs control, n=6 vs 5; Bifidobacterium 16.13% vs 2.49% (p=0.0043).

Mechanism

Not fully elucidated in the source; the in vitro arm suggests a direct effect of progesterone (or a metabolite) on Bifidobacterium growth, independent of host physiology.

Counter-evidence held on record

Pregnancy involves large concurrent changes in estrogen, cortisol, immune function, and diet, any of which could independently affect Bifidobacterium; the human correlation alone would not support causal attribution to progesterone. The mouse-supplementation and in vitro arms substantially strengthen the causal case for progesterone specifically. Reverse-direction (protective) finding, not established in non-pregnant adults on standard HRT/luteal-support dosing. Not a basis for a probiotic-support marketing claim.

Limits on what this shows

• Key negative/null findings: No human ATP-output or cellular energy data in progesterone users were located.
• At-risk subgroups: None identified.

Regulatory status

Not addressed in the FDA label.

Population evidence

Same ovariectomized-rat, isolated whole-brain-mitochondria model as the Mitochondrial burden entry; respiration coupled to ADP phosphorylation was measured directly (oxygen-electrode assay), i.e., bioenergetic/ATP-generating capacity specifically.

Mechanism

Named, animal (Tier B). Progesterone treatment increased metabolic rate/efficiency of ADP-coupled respiration, the step that generates ATP.

Counter-evidence held on record

See Overlap map: this entry, Mitochondrial burden, and Oxidative stress (in part) all rest on the same single animal study and should be read as one data point, not three. Reverse-direction (protective) animal finding only; not a basis for a human "boosts ATP/energy" claim.

Limits on what this shows

• Key negative/null findings: The human study is a within-product, within-subject phase comparison (active-comparator design), not a placebo-controlled test against no hormone exposure, and it used a synthetic progestin-containing combined pill, not progesterone monotherapy; estrogen co-exposure is not excluded as a contributor.
• At-risk subgroups: None identified as harmed; hyperventilation and its clinical significance are context-dependent and not evaluated here as a benefit or harm outside these study contexts.

Regulatory status

Not addressed in the FDA label.

Population evidence

Animal (Tier B): developing rats given progesterone showed increased hypoxic ventilatory response and reduced apnea frequency. Human (Tier C, class-level): women on a triphasic oral contraceptive showed a mean increase in CO2-response slope of 0.78 L/min/mmHg during the high-progestin phase relative to the low-progestin phase of the same product.

Mechanism

Proposed central chemoreceptor sensitization by progesterone/its metabolites; not fully characterized in either source.

Counter-evidence held on record

Per Hard Rule 2, this is the drug's own pharmacologic/mechanistic action (a ventilatory stimulant effect), not a corrected oxygen-delivery deficiency; it is written here as a mechanism clarification for the required category, not as a nutrient-style deficit or benefit claim. Reverse-direction (enhancing) finding; the human evidence is class-level (synthetic progestin, not progesterone) and does not isolate progesterone from concurrent estrogen exposure.