Micronized progesterone
Protects the uterus when estrogen is used.
VIDEO LESSON
How it's taken
• Usually at bedtime, because it can cause drowsiness
• Continuous daily dosing, or cyclic dosing for part of each month
• Taken alongside estrogen when the uterus is present
Side effects at a glance
• Drowsiness or dizziness
• Bloating and breast tenderness
• Irregular bleeding in the first months of continuous use
Monitoring
Bleeding that persists beyond the first 6 months, or starts after a bleed-free period, needs evaluation.
Good to know
• Prometrium capsules contain peanut oil: avoid with peanut allergy
• A levonorgestrel IUD is another way to protect the endometrium (off-label for this use in the US)
AT A GLANCE
Class
Hormone therapy
Brand names
Prometrium
Status
FDA-approved
Reviewed by
Farhan Tahir, MD
Is this medicine right for you?
Prescription medicines need individual medical advice. Dr. Tahir’s team offers consults through Youthful Medicine.
MECHANISM
How it works
Body-identical progesterone, taken as a capsule at bedtime. It binds progesterone receptors in the uterine lining and counters estrogen’s growth signal, preventing the overgrowth (hyperplasia) that can lead to endometrial cancer.
THE EVIDENCE
What the research actually shows.
EVIDENCE SUMMARY
In the PEPI trial, estrogen alone caused endometrial hyperplasia in a large share of women, while adding micronized progesterone kept rates close to placebo. Any woman with a uterus using systemic estrogen needs a progestogen, and the FDA kept that boxed warning in its 2025 label changes.
KEY TRIALS
PEPI
TYPICAL RESULT
Endometrial hyperplasia rates near placebo when paired with estrogen
SAFETY
Risks, side effects and who should avoid it
Can cause drowsiness or dizziness, which is why it is taken at bedtime. The common capsule contains peanut oil, so it is not for people with a peanut allergy. Breast tenderness and irregular bleeding can occur, especially in the first months.
This summary is not a complete list of risks. Read the official prescribing information and discuss your own history with your clinician.
KEY REFERENCE
The Writing Group for the PEPI Trial. Effects of hormone replacement therapy on endometrial histology in postmenopausal women. JAMA 1996;275(5):370–375.
Educational content only, reviewed by Farhan Tahir, MD. Not individual medical advice. Medicines named here require a prescription and medical supervision.