Periods, Hot Flashes and Fatigue on a GLP-1 — What's Known

It's the second month on your GLP-1. The nausea has mostly settled, but your period came ten days early and heavier than usual, you've had a couple of nights waking up hot, and by mid-afternoon you could lie down on the office floor. You search all three at once and find a wall of forum posts, each one certain.

We'd rather be less certain and more accurate. Here's what is and isn't known.

Is fatigue a real side effect?

Yes — this one is on the labels.

	Placebo	Medicine
Wegovy 2.4 mg injection, weight trials	5%	11%
Zepbound 5 / 10 / 15 mg, weight trials	3%	5% / 6% / 7%

Both labels group fatigue with similar terms (asthenia, and for Zepbound lethargy and malaise). The Ozempic label lists fatigue among less common reactions.

Tiredness can also have more ordinary causes when appetite drops: eating little, drinking little, or both. The labels warn that vomiting and diarrhea can lead to dehydration. If you take insulin or a sulfonylurea, the labels also warn about low blood sugar, which can feel like sudden weakness or shakiness.

Can a GLP-1 change my period?

Period changes aren't listed as side effects on the Ozempic, Wegovy, Mounjaro or Zepbound labels. That doesn't settle the question.

A 2026 study in Obstetrics & Gynecology looked at the FDA's adverse-event reporting system for patients aged 12 to 55. With semaglutide, menstrual problems were reported more often than expected — heavy bleeding, bleeding between periods, clots, infrequent periods, and cycles without ovulation. With tirzepatide, bleeding between periods and clots were flagged. Liraglutide, an older GLP-1, showed no signal.

Why does birth control come into this?

Because a late or missed period has one explanation worth ruling out quickly.

The Mounjaro and Zepbound labels say tirzepatide may make oral hormonal contraceptives less effective, because it slows stomach emptying. The effect is largest after the first dose. They advise a non-oral method, or an added barrier method, for 4 weeks after starting and for 4 weeks after each dose increase. The semaglutide labels don't carry this advice.

If you're on tirzepatide and the pill, and your period is late, a pregnancy test is a sensible first step. The Zepbound label says to stop it when a pregnancy is recognized; the Wegovy label says the same for weight reduction.

And hot flashes?

This is the one with the least behind it. Hot flashes aren't listed as a side effect on any of the four labels, and we didn't find good-quality evidence that GLP-1s either cause them or relieve them.

What we do know is that many people start a GLP-1 in their 40s and 50s, when hot flashes are common for other reasons. If they're new for you, perimenopause is worth raising — we cover that in GLP-1 and perimenopause.

How do I tell what's causing what?

Time, mostly. A single heavy period in month two tells you very little. Three months of cycle dates next to dose days, and fatigue rated each day, start to show whether something follows your shots, your cycle, or neither.

That record is useful whatever it shows. If symptoms line up with dose steps, your prescriber has something specific to work with. If they don't, that points the conversation elsewhere.

What we'd track

1. Fatigue with a severity rating. Daily, so you can see whether it peaks after shots or after poor nights.
2. Hot flashes and period changes as custom side effects. Dates and a quick note on flow or intensity.
3. Dose days and dose steps. The backbone for spotting a pattern, and for the tirzepatide contraception window.
4. Water and meals. Low intake is an easy, fixable cause of tiredness.
5. Questions for your clinician. Especially about bleeding or contraception.

Holt's side-effect log has severity sliders for fatigue and lets you add custom side effects, all lined up against dose days. Water and meal logging sit on the same Summary screen.

Log symptoms against your doses in Holt

The doctor-visit report gathers it into one page with suggested questions. Bring your cycle tracker alongside it, and the conversation gets a lot more specific.