Every month, for two or three days, you feel like you are coming down with something. Your body aches the way it aches with flu. You are cold, then clammy. You are exhausted in a way that sleep does not fix. Your gut turns on you. Then you bleed, and within a day or two it lifts — and because it lifts, nobody, including you, takes it seriously.
They should. "Period flu" is not a diagnosis and it is not an infection — no virus, no contagion, nothing to catch. But almost every symptom in it maps onto a specific, measurable biological event. Two of them, in fact. This page is the mechanism; the individual symptoms have their own pages, linked below.
The two mechanisms, properly explained
1. Prostaglandins do not stay in your uterus
As progesterone falls at the end of the luteal phase, the endometrium begins to break down and releases prostaglandins — signalling lipids, principally PGF2a and PGE2. Their job is local: make the myometrium contract, squeeze the spiral arteries, shed the lining. That is what a cramp is.
But prostaglandins are not confined to the pelvis. They diffuse into the circulation, and their receptors sit on smooth muscle and vascular endothelium everywhere. So the same chemical that is contracting your uterus is also acting on your intestine, your stomach, your blood vessels and your pain-signalling nerves. This is not a theory invented to explain away women's complaints — it is the accepted pathophysiology of primary dysmenorrhoea. It is why ACOG names prostaglandins as the cause of period pain, and why NSAIDs are first-line: they inhibit the cyclo-oxygenase enzymes that make prostaglandins in the first place.
The dose-response matters. Work going back to Dawood's foundational research shows that women with more severe menstrual symptoms have higher measured prostaglandin levels in menstrual fluid, and that severity tracks with those levels. If your period flu is worse than your sister's, the most likely explanation is that you are producing more of the chemical — not that you are less stoic.
2. Estrogen and progesterone withdrawal
In the five or so days before bleeding, both hormones drop steeply. Estrogen is not only a reproductive hormone: it modulates serotonin synthesis and receptor sensitivity, influences the hypothalamic thermoregulatory set point, and shapes sleep architecture. Progesterone's metabolite allopregnanolone acts on GABA receptors — the same system targeted by sedatives — so its withdrawal is a withdrawal from your body's own calming signal.
Pull both away over a few days and you get the second half of the syndrome: flattened mood, wired-but-tired insomnia, temperature instability (the chills, the sudden sweats), headache, and a fatigue that has nothing to do with how much you slept. This is the same axis that drives PMS and, at its severe end, PMDD. Importantly, PMDD is not caused by abnormal hormone levels — levels are typically normal. It is an abnormal sensitivity to normal hormonal change, which is exactly why "your bloods are fine" is not a refutation of your symptoms.
Symptom by symptom: which mechanism is doing what
| Symptom | Main driver | What is physically happening | Read more |
|---|---|---|---|
| Cramps and whole-body aching | Prostaglandins | Myometrial contraction plus vasoconstriction of uterine vessels; circulating prostaglandins also sensitise pain nerves — the identical pathway that makes you ache during a real infection | Period pain |
| Lower back and thigh ache | Prostaglandins | Uterine pain is referred along shared spinal segments to the low back and thighs; deep, constant back pain that outlasts the bleed is a different signal | Period back pain |
| Nausea, loss of appetite, vomiting | Prostaglandins | Effects on gastric smooth muscle and stimulation of nausea pathways; worse when prostaglandin output is high | Period nausea |
| Diarrhoea, urgency, loose stools | Prostaglandins | The same smooth-muscle receptors line the bowel; prostaglandins increase intestinal motility and fluid secretion | Gut symptoms |
| Lightheadedness, feeling faint on standing | Prostaglandins (plus blood loss) | Vasodilation lowers peripheral resistance and blood pressure; heavy bleeding and low iron compound it | Period dizziness |
| Bone-deep fatigue, poor sleep, low mood, irritability | Hormone withdrawal | Estrogen's effects on serotonin and sleep architecture, plus loss of progesterone-derived allopregnanolone at GABA receptors | Period fatigue |
| Chills, hot flushes, temperature swings | Hormone withdrawal | Falling estrogen destabilises the hypothalamic thermoregulatory set point — chills and sweats without a measured fever | Hot flushes |
| Headache or migraine just before bleeding | Hormone withdrawal | Estrogen withdrawal is a recognised migraine trigger; prostaglandins add a vascular and inflammatory component | Hormonal migraine |
| Needing to pee constantly in the days before | Both | Fluid shifts, prostaglandin effects on bladder smooth muscle, and a bulkier premenstrual uterus pressing on the bladder — no burning, no fever | Frequent urination before your period |
| Bloating, breast tenderness, swelling | Hormone withdrawal | Fluid shifts and mammary tissue response to the luteal hormone profile | Water retention |
Notice what this table does: it turns "I feel ill and I don't know why" into a list of separate physiological events with a common origin. That is the point. You are not having ten unrelated problems and you are not catastrophising — you are downstream of one endocrine event with wide receptor distribution.
What actually helps — and why
NSAIDs work because of the mechanism, not in spite of it. Ibuprofen, naproxen and mefenamic acid inhibit cyclo-oxygenase, so the endometrium makes fewer prostaglandins in the first place. A Cochrane review of 80 randomised trials in 5,820 women found NSAIDs clearly more effective than placebo for period pain — and, in the same review, more likely than placebo to cause side effects. That is an explanation of why they help, not a recommendation that you take them: they carry real risks (stomach, kidney, cardiovascular, drug interactions), they are not suitable for everyone, and whether and how you use them is a decision for you and your prescriber or pharmacist. If you want the drug detail, see our page on ibuprofen, and check anything you already take with the interaction checker.
Heat. A hot water bottle or heat patch on the lower abdomen increases local blood flow and works partly through the same gate-control pathways as pain relief. It is cheap, low-risk, and does not compete with anything else you are doing — more options in menstrual cramp relief.
Sleep and load. Estrogen withdrawal is already degrading your sleep in the exact window when your body is running an inflammatory-feeling process. Protecting sleep in the late luteal phase is not a soft suggestion; it is the only lever that acts directly on the fatigue arm of the mechanism. Track two or three cycles with the period tracker and you can see the bad days coming — and stop scheduling your hardest week into them.
Hormonal contraception is sometimes prescribed to flatten the cycle: it thins the endometrium (less tissue, fewer prostaglandins) and removes the sharp hormone withdrawal. It genuinely helps some women and is unsuitable for others; it is a prescriber's decision, not a self-treatment.
What does not deserve your money: nothing on the supplement shelf reliably lowers endometrial prostaglandin production. Some women report partial benefit from magnesium or omega-3, and that is fine — but it is a small effect layered on top of the two mechanisms above, not a fix for them.
The midlife angle nobody writes about
Here is the thing you will not find on a page written for a 25-year-old. If you are in your forties and you have suddenly developed period flu you never used to get, you are most likely looking at perimenopause — not a new illness.
Perimenopause is not a gentle downward slope of estrogen. Research on the menopause transition describes it as erratic: larger day-to-day swings, higher peaks and lower troughs than in a regular premenopausal cycle. Add increasingly frequent anovulatory cycles, where progesterone barely rises, and cycles in which a thickened endometrium sheds a heavier bleed.
The consequences follow logically from the mechanisms:
- The withdrawal is steeper — you are falling from a higher estrogen peak, so the mood crash, chills, headache and insomnia hit harder.
- Heavier bleeds mean more endometrium shed, and more prostaglandin — so cramps, gut symptoms and body aches can be worse in your forties than they ever were in your twenties.
- It becomes unpredictable — a brutal month, then a mild one. That erratic pattern is itself a perimenopause signal, and it is precisely why women assume something new and frightening is wrong.
- Blood loss compounds it — heavier periods drain your iron stores, and iron deficiency causes its own fatigue, breathlessness and lightheadedness that stack on top of the cycle symptoms. Ferritin (your iron store) falls before haemoglobin does, so a "normal" full blood count does not rule it out — ferritin has to be measured specifically. This is worth a blood test, not a guess.
If that is you, the useful next reads are perimenopause symptoms and irregular periods in perimenopause, and the menopause stage quiz will tell you whether your pattern fits the transition.
When to see a doctor
The single most damaging idea in this area is that severe cyclical symptoms are something to endure because they are "just hormones." Severity is a clinical signal, not a personality test. Get medical assessment if:
- You have a true fever. A measured temperature of 38°C / 100.4°F or above is not period flu. Prostaglandin release causes chills, sweats and the subjective sense of running a temperature — not documented pyrexia. Fever with pelvic pain or abnormal discharge can mean infection, including pelvic inflammatory disease. Sudden severe pain with fever, or with fainting, needs urgent care the same day.
- The symptoms stop your life — days off work, plans cancelled every month, pain that over-the-counter analgesia does not touch. The NHS is explicit that period pain which does not respond to usual treatment should be reviewed. That threshold should trigger investigation for endometriosis or adenomyosis, both of which are routinely missed for years.
- The mood component is severe — if luteal-phase depression, rage or hopelessness is disproportionate and reliably lifts when you bleed, that pattern is PMDD. It is a recognised diagnosis with real treatments, and prospective symptom charting across two cycles is what gets it recognised.
- The fatigue, cold intolerance and aching are not confined to the luteal phase — that points away from the cycle and towards thyroid disease or anaemia. Both are simple blood tests. The fatigue cause finder will help you work out what to ask for.
- Bleeding has become heavy or the pattern has changed sharply — soaking through a pad or tampon every hour or two, clots bigger than a 10p coin or a quarter, bleeding between periods, or any bleeding after 12 months without a period. All of these need evaluation.
A note on being believed. Cyclical symptoms are among the most dismissed in medicine, partly because they have resolved by the time the appointment comes round. The fix is data: chart your symptoms and their severity against cycle day for two or three months, and take the chart with you. A pattern on paper is far harder to wave away than "I feel awful before my period" — and it is exactly the evidence a clinician needs to separate period flu from PMDD, endometriosis, thyroid disease or iron deficiency.
Your body is not betraying you and you are not exaggerating. You are experiencing the systemic effects of a chemical your uterus makes on purpose, plus the withdrawal of two hormones your brain runs on. That deserves an explanation — and, if it is stopping your life, an investigation.
