Inclusion criteria - women with PCOS:

1. Age 18 to 40 years
2. Body mass index (BMI) ≥25 to ≤40 given that 95% of all women with PCOS with a BMI ≥25 are insulin resistant (71,72).
3. PCOS diagnosis according to Rotterdam criteria 2003 (73), with at least two of the following three symptoms: Clinical signs of hyperandrogenism (hirsutism or acne); oligo/amenorrhea; and/or polycystic ovaries (PCOS). Hirsutism is defined as a self-reported Ferriman-Gallwey (FG) score ≥8 (≥5 Asian) (74,75). Acne is defined by a positive response to the question Do you have acne? Oligomenorrhea is defined as an intermenstrual interval >35 days and <8 menstrual bleedings in the past year. Amenorrhea as <3 cycles per year. PCO is defined by transvaginal ultrasound with ≥12 follicles 2-9 mm and/or ovarian volume ≥10 ml in one or both ovaries.
4. Willing to sign the consent form.

Inclusion criteria - controls:

Controls should have BMI >25 to <40, regular cycles with 28 days ± 2 days, and no signs of hyperandrogenism. They are excluded if they have menstrual irregularities, signs of hyperandrogenism (FG >4), or evidence of PCO morphology on ultrasound.

Exclusion criteria for all women

1. Age >40
2. Exclusion of other endocrine disorders such as non-classic congenital adrenal hyperplasia (17-hydroxyprogesterone < 3nmol/L), androgen secreting tumors or suspected Cushing's syndrome.
3. Having known renal disease (creatinine clearance < 60 mL/min), hepatic insufficiency, autoimmune disorders or cancer.
4. Any acute condition with potential to alter renal function or cause tissue hypoxia.
5. Type I diabetes.
6. Pharmacological treatment (cortizon, antidepressant, other antidiabetic treatment such as insulin and acarbose, hormonal contraceptives, hormonal ovulation induction or other drugs judged by discretion of investigator) within 12 weeks. Depo Provera or similar within 6 months.
7. Hypersensitivity to metformin hydrochloride or to any of the excipients.
8. Blood pressure >160 / 100 mmHg
9. Pregnancy or breastfeeding the last 6 months
10. Acupuncture the last 2 months
11. Daily smoking and alcoholic intake
12. Language barrier or disabled person with reduced ability to understand the information given.

In total 50 controls will be matched at baseline (age, weight and BMI) to women with PCOS. Controls will undergo screening and baseline visit, but will not be randomized to any treatment.