Periods on a weight loss jab: what changes, and when to worry

SASian Anderson, Writer and editor, MyGLP. Sources checked 21 October 2026.

Weight loss medicines do not act on your ovaries. They act on appetite, blood sugar and how fast your stomach empties. The changes women notice in their periods come from the weight loss itself, from eating less, and from the shift in hormones that follows both.

Why your cycle moves

Fat tissue makes oestrogen. Carrying extra weight can push oestrogen up and knock ovulation off course, which is one reason heavier women often have irregular or missed periods. As weight comes off, oestrogen levels settle and ovulation can start again. For some women that means a period after months or years without one. For others it means a cycle that was long and unpredictable becomes shorter and regular.

The second driver is energy. If you eat far less than your body needs for a stretch of weeks, the brain can pause the signals that trigger ovulation. Periods go light, late or stop. This is the same thing athletes and women with eating disorders see, and it reverses when intake rises. The NHS page on missed periods lists sudden weight loss alongside stress and over-exercise as common causes.

What women report

• A period arriving earlier or later than usual in the first two or three months.
• Heavier bleeding for a cycle or two as ovulation restarts.
• Lighter periods, or a missed month, when intake drops sharply during a dose increase.
• Worse cramps or PMS for a while, then settling.
• A period after a long gap, which can be a surprise for women with PCOS.

The medicine leaflets for Mounjaro and Wegovy do not list period changes as a side effect, because the trials did not measure them. The reports come from women, GPs and pharmacists, and they are common enough to expect.

The ovulation surprise

If ovulation comes back, you can get pregnant. That applies even when your periods have been absent for years. The NHS advises contraception while you take these medicines, and extra care with Mounjaro: use a barrier method as well as the pill for 4 weeks after starting and for 4 weeks after each dose increase, because the slower stomach can reduce how much of the pill you absorb. Our guide to contraception on a weight loss jab covers the detail.

When to call your GP

Most cycle changes settle within three months as your weight loss steadies. Speak to your GP if any of these apply:

• No period for 3 months or more and you are not pregnant and not on a contraceptive that stops periods.
• Bleeding between periods or after sex.
• Bleeding after the menopause.
• Periods so heavy you soak through a pad or tampon every hour for several hours, or pass clots larger than a 10p coin. The NHS heavy periods page lists these as reasons to be seen.
• Pain that stops you doing normal things.

Heavy periods also drain iron. If you feel tired, short of breath on stairs or dizzy when you stand, ask for a blood test. Our guide on iron and tiredness explains what the numbers mean.

Keeping track

Note the first day of each period, how heavy it was and how you felt. A free period app or a line in your phone notes is enough. Three months of dates tells a GP far more than a vague "it has been all over the place". It also shows you whether a change lines up with a dose increase, a week of eating very little, or neither.

Sources

1. Stopped or missed periods (NHS)
2. Heavy periods (NHS)
3. Tirzepatide: side effects, pregnancy and contraception (NHS)
4. Wegovy patient information leaflet (Novo Nordisk, via the electronic medicines compendium)
5. GLP-1 agonists and oral contraception, statement February 2025 (Faculty of Sexual and Reproductive Healthcare)