Metformin is a diabetes medicine that doctors also prescribe for PCOS, now officially renamed PMOS. The most common metformin side effects in women are stomach upset, nausea, diarrhea, gas and a metallic taste, mostly in the first few weeks. Taking it with food, starting at a low dose and the extended-release form often help. Long-term use can lower vitamin B12. A rare but serious reaction needs urgent care; the warning signs are listed below.
Why metformin is prescribed for PCOS
Many women with PCOS have insulin resistance, even at a normal weight. High insulin pushes the ovaries to make more testosterone. It also lowers SHBG, a protein that binds testosterone, so more of it is free to act. That can mean irregular periods, acne and extra hair growth. Metformin lowers insulin. Some women find their cycles become more regular. Effects on weight and skin are usually modest.
One visible clue: dark, velvety patches of skin on the neck or under the arms (acanthosis nigricans) are a sign of insulin resistance. If you notice them, ask your doctor about a blood sugar test.
Common side effects
- nausea, stomach upset or loss of appetite
- diarrhea, gas or bloating
- a metallic taste in the mouth
These usually ease within a few weeks as your body adjusts.
How to make metformin easier to take
- Take it with meals, or as your prescription directs.
- Start low and increase slowly. Doctors usually build the dose up over several weeks.
- Ask about the extended-release form. Many women find it gentler on the stomach.
- Don’t change your dose on your own. If side effects continue, your prescriber can adjust the plan.
Long-term use: B12 and kidney checks
Metformin can lower vitamin B12 over time. Ask whether your levels should be checked, especially if you notice tingling in your hands or feet, or unusual tiredness. Your doctor will also check kidney function from time to time.
Pregnancy
Metformin can help ovulation return, so pregnancy may become possible sooner than you expect. Use contraception if you aren’t planning a pregnancy. If you become pregnant, tell your doctor promptly, and don’t stop the medicine on your own.
Get help promptly
- Severe vomiting, diarrhea, fever or dehydration: call your doctor, as metformin may need to be paused until you recover.
- Before surgery or a scan with injected contrast dye: tell the team you take metformin.
- Seek urgent care for unusual weakness with vomiting or stomach pain, fast or difficult breathing, marked dizziness, or feeling unusually cold. These can be signs of lactic acidosis, a rare but serious reaction.
Other options your doctor may discuss
Metformin is one of several PCOS treatments. Depending on your goals, your doctor may also discuss birth control pills for cycle regularity, spironolactone for acne and hair growth, inositol, or lifestyle changes; see how to regulate periods naturally. If metformin remains hard to take, ask what else addresses the same goal. Our PCOS care-plan guide covers the questions to bring.
An external routine alongside your care
If you’d like a routine that involves nothing swallowed, CycleBalance is one option to add to your plan. It is a sealed herbal pouch worn over the navel, held by a reusable belt and replaced every five days. It does not replace metformin, and it does not treat insulin resistance or PCOS.
In Dr. Wang’s practice records from 2016 to 2022, seven women with irregular cycles used the CycleBalance formula. One woman in her early 30s, whose cycles had run 39 to 42 days, reported a 35-day cycle after her course. These were selected, uncontrolled cases, so they cannot show what caused the change or predict your result. Review the ingredient list with your doctor or pharmacist.
See CycleBalance’s directions →
Reviewed October 2026. Author: Qingcai Wang, MD, PhD, developer of the Five Essences external herbal-pouch formulations, with a commercial interest in their sale. General education only; continue your medical care.
Sources: NHS: metformin; MedlinePlus: metformin; 2023 International PCOS (PMOS) Guideline.