For a tension headache, 1,000 mg did nothing at the one-hour mark
Across 23 trials and 8,079 adults, 1,000 mg of paracetamol was no different from placebo at one hour. By two hours, 22 people had to take it for one more to be pain-free.
What the study found
Twenty-two people take paracetamol so one more is headache-free
That is the number for the outcome headache researchers care most about, being pain-free two hours in, across 23 trials and 8,079 adults on high-quality evidence. At one hour paracetamol 1,000 mg was no different from placebo. Doses of 500 to 650 mg were not better than placebo either.
The caveat · Not migraine, not chronic headache
This is frequent episodic tension-type headache of moderate or severe intensity, not migraine, which the trials generally excluded, and not the chronic form. Ask an easier question and the number improves: for pain-free or only mild pain at two hours it is 10, and for avoiding a second painkiller it is 7.8. No trial was at low risk of bias on every count, though the reviewers put most of that down to thin reporting rather than bad conduct. The senior author declares grants and fees from analgesic manufacturers.