Why Period Cramps Happenand What the Evidence Says Helps

Period cramps are so common that they are often waved away as something to put up with. Yet for many women they are severe enough to keep them home from school or work, and sometimes they are the first sign of a condition that benefits from an early diagnosis.

This article explains why cramps happen, the difference between ordinary period pain and pain with an underlying cause, what the clinical trials say about the things people reach for, and the warning signs that mean it is time to see a clinician. It is educational, not personal medical advice.

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How Common Period Cramps Are

Very. The American College of Obstetricians and Gynecologists (ACOG) says more than half of women who menstruate have some pain for 1 to 2 days each month, usually mild, but for some severe enough to stop normal activities for several days.

In younger women the numbers are higher. A 2019 systematic review and meta-analysis in the Journal of Women's Health pooled 38 studies covering 21,573 women under 25 and found that 71.1% reported dysmenorrhea, with no real difference between richer and poorer countries. One in five (20.1%) said period pain had kept them away from class, and 40.9% said it hurt their concentration or performance. A 2015 review in Human Reproduction Update put the range across studies at 45% to 95%, and described primary dysmenorrhea as common, often debilitating, and often poorly managed because it is regarded as normal.

Primary and Secondary Dysmenorrhea

Clinicians divide period pain into two kinds, and the distinction shapes everything that follows.

Primary dysmenorrhea

This is the cramping pain that comes just before or during a period in an otherwise healthy uterus. ACOG notes that it usually begins soon after periods start, often becomes less painful with age, and may ease after giving birth. The pain is felt low in the abdomen, sometimes spreading to the lower back and thighs, and is worst on the first day or two of bleeding. For some women it comes with nausea, diarrhea, headache or dizziness.

Secondary dysmenorrhea

This is period pain caused by a disorder of the reproductive organs. ACOG lists endometriosis, uterine fibroids and adenomyosis as the leading causes, along with structural differences present from birth and conditions such as Crohn's disease that can flare during a period. The NHS adds pelvic inflammatory disease and copper or hormonal IUDs, especially in the first 3 to 6 months after fitting.

The pattern is the giveaway. Secondary pain tends to get worse over time rather than better, often starts a few days before bleeding, lasts longer than ordinary cramps, and may not stop when the period ends. We come back to this in the red flags section below.

Why Cramps Happen: The Role of Prostaglandins

The muscle of the uterus contracts throughout the cycle, but around menstruation those contractions become stronger. The trigger is a group of hormone-like chemicals called prostaglandins, made in the lining of the uterus as it prepares to shed. ACOG describes the sequence plainly: prostaglandins cause the muscles and blood vessels of the uterus to contract, their level is highest on the first day of a period, and as the lining is shed the level falls, which is why pain usually eases after the first day or two.

Strong contractions briefly squeeze the blood vessels in the uterine wall, reducing blood flow and oxygen to the muscle, and that short-term lack of oxygen is a large part of what you feel. Prostaglandins also travel in the bloodstream, which is why the same chemistry can bring nausea, loose stools and headache along with the pain.

The 2015 Cochrane review of anti-inflammatory drugs for dysmenorrhea notes that women with dysmenorrhea have been shown to have high levels of prostaglandins, which is the rationale for the most widely used medication. The 2015 Human Reproduction Update review adds a less obvious finding: women with dysmenorrhea show greater sensitivity to experimental pain even at times of the cycle when they are not in pain, suggesting that repeated monthly pain may change how the nervous system processes it. That is one argument against simply enduring severe cramps year after year. For where this sits in the month, see the four phases of the menstrual cycle.

What the Evidence Says Helps

Primary dysmenorrhea is one of the better-studied areas of women's health. Here is what the strongest sources say, with the quality of the evidence noted.

Anti-inflammatory painkillers (NSAIDs)

Nonsteroidal anti-inflammatory drugs such as ibuprofen and naproxen work by blocking prostaglandin production, which makes them a direct answer to the cause. The 2015 Cochrane review pooled 80 randomized trials with 5,820 women. NSAIDs were clearly more effective than placebo: the review estimates that if 18 in 100 women taking a placebo get moderate or excellent pain reduction, between 45 and 53 in 100 taking an NSAID will. They also beat acetaminophen (paracetamol) in the three trials that compared them.

The trade-off is side effects. NSAIDs raised the odds of stomach upset and of headache and drowsiness; the review estimates that if 10 in 100 women on placebo get side effects, 11 to 14 in 100 on NSAIDs will. No single NSAID stood out, the evidence was rated low quality for most comparisons, and 59% of trials were commercially funded. ACOG's advice is to take an NSAID at the first sign of pain or bleeding, usually for only 1 to 2 days, and to avoid them if you have a bleeding disorder, asthma, aspirin allergy, liver damage, stomach problems or ulcers.

Heat

A hot water bottle on the belly is the oldest approach there is, and it has been tested. A 2018 systematic review and meta-analysis in Scientific Reports found six randomized trials of heat for primary dysmenorrhea. Two found heat better than an unheated placebo, one found it better than no care at all (a reduction of about 4 points on a 10-point pain scale), and in two pooled trials heating pads reduced pain more than analgesic medication did. The authors called the evidence suggestive rather than robust, because the trials were few and small. Both ACOG and the NHS list a warm bath, a heat pad or a hot water bottle among the home measures worth trying.

Exercise

The 2019 Cochrane review of exercise for dysmenorrhea included 12 trials with 854 women, almost all comparing exercise with doing nothing. Exercise, whether low intensity (stretching, core work, yoga) or high intensity (aerobic classes), may have a large effect on pain, equivalent to about 25 mm on a 100 mm scale, which the authors consider clinically meaningful. The catch is that the evidence is low quality: the studies were small, their results varied widely, and blinding was impossible.

Two details are useful in practice. The programs that worked involved about 45 to 60 minutes of exercise, three or more times a week, throughout the month rather than during the period itself. And the reviewers could not say whether the benefit lasts once regular exercise stops, or whether it applies equally to women over 25.

Ginger

Ginger is the one kitchen ingredient with a run of randomized trials for period pain. A 2016 systematic review in Evidence-Based Complementary and Alternative Medicine found six trials, all from Iran or India. In three placebo-controlled trials, oral ginger reduced pain by an average of about 1.5 cm on a 10 cm scale; in two trials against the NSAID mefenamic acid, there was no significant difference between them. A separate 2015 meta-analysis in Pain Medicine reached a similar conclusion from four placebo-controlled trials, describing suggestive evidence for 750 to 2,000 mg of ginger powder a day during the first 3 to 4 days of the period. A 2021 meta-analysis in Cureus pooling five placebo trials again found ginger better than placebo for pain severity, but not for how long the pain lasted.

All three reviews rate the evidence as low quality: small trials, mostly single-blind, high variability between studies, and no standardization of the ginger itself. Reported side effects were mild, mostly heartburn. The trials used ginger powder in capsules, so a cup of ginger tea is not the same dose, and the research cannot tell you what a tea delivers.

TENS machines

Transcutaneous electrical nerve stimulation uses sticky pads on the skin to deliver mild electrical pulses. The 2024 Cochrane review included 20 trials with 585 women and concluded that both high-frequency and low-frequency TENS may reduce pain compared with placebo or no care, with certainty downgraded to low because of risk of bias. A 2022 meta-analysis in Explore of four sham-controlled trials (260 women) also favored real TENS. The NHS lists TENS among the options a GP may recommend, and ACOG mentions nerve stimulation alongside acupuncture and acupressure.

Hormonal birth control

For women not trying to conceive, ACOG notes that combined pills, patches and rings, progestin-only implants and injections, and the hormonal IUD can all be used for painful periods, with bleeding often getting lighter the longer an IUD is in place. This is a prescription decision with its own trade-offs.

Vitamins and minerals

ACOG's position is that vitamin B1 or magnesium supplements may be helpful, but that there has not been enough research to recommend them.

The Evidence at a Glance

Warmth and Tradition Around Your Period

Much of the evidence above lines up with what grandmothers in many cultures already did: warmth on the belly, something hot and gingery to drink, keeping moving, and resting when the first day is bad. In Chinese tradition the days around a period are a time to keep warm, avoid cold or raw food and drink, and sip warm infusions of ginger, red dates, longan and brown sugar. The explanation given is one of cold and stagnation in the lower body, a different framework from prostaglandins, but the habits it produces are mostly the ones the trials have looked at. The tradition's formal answer to that pattern is Wen Jing Tang, a recipe recorded around 200 CE that pairs cinnamon twig, ginger and evodia with blood-nourishing roots such as dang gui and white peony; our article on Traditional Chinese Medicine and the cycle reviews its trial record.

None of that makes the traditional foods a proven therapy. The ginger trials used measured capsules, not tea, and the other ingredients have little or no clinical research behind them for period pain. What the tradition offers is a calm, warm, food-based routine for a few days a month. We describe the thinking behind it in how Traditional Chinese Medicine views the menstrual cycle, and the ingredients, along with what is and is not known about them, in foods and teas traditionally taken around your period.

Red Flags: When Cramps Are Something More

Most period pain is primary dysmenorrhea and does not signal disease. But some women with painful periods have an underlying condition, and the earlier it is identified the better. The one that matters most is endometriosis.

Endometriosis

In endometriosis, tissue similar to the lining of the uterus grows outside it, on the ovaries, fallopian tubes, bowel, bladder or the lining of the pelvis, and bleeds in response to the same hormones, causing inflammation, pain and scar tissue. ACOG estimates that about 1 in 10 women of reproductive age develop it, and that it is a common cause of infertility. The symptoms it lists include period pain that gets worse over time, chronic pelvic pain, pain during sex, pain with urination or bowel movements, and bloating or constipation. The amount of pain does not always match the extent of the disease.

Other causes

• Adenomyosis: uterine lining tissue growing into the muscle wall of the uterus, more common in women who have had children.
• Fibroids: non-cancerous growths in or on the uterus. Small ones usually cause no pain; larger ones in the wall can.
• Pelvic inflammatory disease: infection of the reproductive organs.
• An IUD: especially in the first months after fitting.

The pattern to watch for, according to both ACOG and the NHS: pain that is new in adulthood after years of easy periods, pain that is getting steadily worse, pain that starts days before bleeding and outlasts it, and pain that comes with bleeding between periods, pain during sex, or pain when passing urine or stool. If that describes you, a hot water bottle is not the answer, and it deserves a diagnosis. Period pain can also change after childbirth; our guide to your first period after birth covers what to expect.

When to Talk to a Clinician

ACOG's advice is simply that if you have painful periods, you and your ob-gyn should talk about them. The NHS and ACOG guidance points to these situations as the ones not to leave:

• Pain that stops you doing your normal daily activities, or that over-the-counter painkillers and heat do not control.
• Periods that are becoming more painful, heavier or more irregular than they used to be.
• Pain during sex, or when urinating or passing stool.
• Bleeding between periods, a swollen abdomen, loss of appetite or unexplained weight loss.
• Severe pelvic pain that is suddenly worse than usual and not responding to painkillers. The NHS classes this as urgent; in the US, call your clinician or go to urgent care.
• Difficulty conceiving alongside painful periods, since endometriosis is involved in a large share of infertility.

Expect your clinician to ask about the timing and pattern of the pain, possibly to do a pelvic exam, and if medication has not helped, to arrange an ultrasound. ACOG notes that if medication fails, the focus should shift to finding the cause. A simple record of pain scores, bleeding days and what you took is the most useful thing you can bring to that appointment. If premenstrual mood and physical symptoms are a bigger problem than the cramps themselves, see our companion guide to what PMS is and what the research says.

Frequently Asked Questions

What causes period cramps?

Period cramps are caused by prostaglandins, natural chemicals made in the lining of the uterus as it prepares to shed. They make the uterine muscle contract and briefly squeeze its blood vessels, which is felt as cramping. Prostaglandin levels are highest on the first day of a period and fall as the lining is shed, which is why the pain usually eases after a day or two.

What works best for period cramps?

The strongest evidence is for anti-inflammatory painkillers such as ibuprofen and naproxen, which block prostaglandins; a Cochrane review of 80 trials found they roughly triple the proportion of women who get good pain reduction compared with placebo. Heat on the abdomen, regular exercise through the month, ginger powder and TENS machines each have smaller bodies of lower-quality trial evidence suggesting they also reduce pain.

When are period cramps a sign of something serious?

See a clinician if period pain is getting worse over time, starts days before bleeding and outlasts it, stops you doing normal activities, or comes with pain during sex, pain when urinating or passing stool, or bleeding between periods. These patterns can point to endometriosis, which affects about 1 in 10 women of reproductive age, or to adenomyosis, fibroids or pelvic infection.

References

1. American College of Obstetricians and Gynecologists. Dysmenorrhea: Painful Periods. Frequently Asked Questions. ACOG

2. American College of Obstetricians and Gynecologists. Endometriosis. Frequently Asked Questions. ACOG

3. National Health Service. Period pain. Reviewed March 2026. NHS

4. Armour, M. et al. (2019). The Prevalence and Academic Impact of Dysmenorrhea in 21,573 Young Women: A Systematic Review and Meta-Analysis. Journal of Women's Health, 28(8), 1161 to 1171. PubMed

5. Iacovides, S., Avidon, I. & Baker, F.C. (2015). What we know about primary dysmenorrhea today: a critical review. Human Reproduction Update, 21(6), 762 to 778. Oxford Academic

6. Marjoribanks, J. et al. (2015). Nonsteroidal anti-inflammatory drugs for dysmenorrhoea. Cochrane Database of Systematic Reviews, 7, CD001751. PubMed

7. Jo, J. & Lee, S.H. (2018). Heat therapy for primary dysmenorrhea: A systematic review and meta-analysis of its effects on pain relief and quality of life. Scientific Reports, 8, 16252. PubMed

8. Armour, M. et al. (2019). Exercise for dysmenorrhoea. Cochrane Database of Systematic Reviews, 9, CD004142. PubMed

9. Chen, C.X., Barrett, B. & Kwekkeboom, K.L. (2016). Efficacy of Oral Ginger (Zingiber officinale) for Dysmenorrhea: A Systematic Review and Meta-Analysis. Evidence-Based Complementary and Alternative Medicine, 2016, 6295737. PMC

10. Daily, J.W. et al. (2015). Efficacy of Ginger for Alleviating the Symptoms of Primary Dysmenorrhea: A Systematic Review and Meta-analysis of Randomized Clinical Trials. Pain Medicine, 16(12), 2243 to 2255. PubMed

11. Negi, R. et al. (2021). Efficacy of Ginger in the Treatment of Primary Dysmenorrhea: A Systematic Review and Meta-analysis. Cureus, 13(3), e13743. PubMed

12. Han, S. et al. (2024). Transcutaneous electrical nerve stimulation (TENS) for pain control in women with primary dysmenorrhoea. Cochrane Database of Systematic Reviews, 7, CD013331. PubMed

13. Arik, M.I. et al. (2022). The effect of TENS for pain relief in women with primary dysmenorrhea: A systematic review and meta-analysis. Explore, 18(1), 108 to 113. PubMed