Progesterone: what it treats, how to take it, side effects
Last updated September 3, 2026.
Progesterone is a progestin - the hormone paired with estrogen in menopausal hormone therapy to protect the uterus. You may know it as Prometrium. It also brings on a period in women whose cycles have stopped for lack of it.
What does it treat?
Two jobs: alongside estrogen in menopausal hormone therapy (MHT), where it balances estrogen's effect on the uterus - and to produce a period in women with amenorrhea, replacing the natural progesterone that is missing.
How do you take it?
A capsule once a day in the evening or at bedtime - bedtime matters, since dizziness and drowsiness are common early - typically for 12 days of each 28-day cycle, exactly on the schedule your doctor sets. Same time every evening. Missed dose: take it when you remember unless the next one is close; never double up.
Side effects to know
- Common: headache, breast tenderness or pain, upset stomach or belly pain, tiredness, mood swings, irritability or excessive worrying, vaginal discharge, and swelling of arms or legs.
- Serious - call your doctor immediately or seek emergency care: sudden swelling or pain in an arm or leg; chest pain, shortness of breath, or coughing up blood; the stroke set - face drooping, speech difficulty, severe headache, arm weakness, vision changes, dizziness, confusion, numbness, trouble walking; breast lumps; migraine; bulging eyes or double vision; unexpected vaginal bleeding; and the allergic set - hives, rash, itching, trouble breathing or swallowing, facial or throat swelling.
Who should not take it?
Flag history of blood clots, stroke, heart attack, breast cancer, liver disease, and unexplained vaginal bleeding - each can rule hormone therapy out or reshape it. Peanut allergy: the capsules contain peanut oil - say it before the first dose. If you become pregnant, call your doctor.
When is it an emergency?
Chest pain, coughing blood, face drooping or speech trouble, or sudden vision change is a 911 call. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
What a Pymander AI doctor consult looks like
Illustrative example, not a real member's messages.
Common questions
Why do I take it only 12 days a month?
The cycling mimics the body's own rhythm: estrogen runs all month, progesterone joins for part of it - in the 12-days-per-28 pattern - to balance estrogen's effect on the uterine lining. That balance is the uterus-protection job. Your doctor sets the exact pattern; the calendar on the fridge is the compliance tool.
Why at bedtime?
Dizziness and drowsiness are common, especially early - bedtime dosing turns the side effect into a sleep aid and keeps it out of your day. Same time every evening keeps the level steady. If morning grogginess hangs on, tell your doctor - timing and formulation have options.
Why does estrogen need a partner?
Estrogen alone stimulates the uterine lining - unopposed, that raises the lining-cancer risk in anyone with a uterus. Progesterone is the counterweight. No uterus (after hysterectomy), and the equation changes - which is why hormone therapy is prescribed to the person, not the symptom.
What unexpected bleeding means on MHT?
Scheduled withdrawal bleeding can be part of the plan; bleeding outside the pattern is the signal. Unexpected vaginal bleeding is on the call-your-doctor list because the lining deserves a look when it speaks up - usually benign, always worth the check. Note the day and the pattern before the call; that detail speeds the answer.
Why did the pharmacist ask about peanut allergy?
Prometrium capsules contain peanut oil - a genuine allergy flag, not a formality. A true peanut allergy changes the prescription. If you have ever reacted to peanuts and nobody asked, say it before the first capsule.
What happens if I miss a dose of progesterone?
Take it as soon as you remember, unless it is almost time for the next one - then skip it and never double up. On the 12-day cycle, a missed day can nudge the bleeding pattern - note it, and if the cycle that follows looks off, that note is what your doctor wants to hear.