Abstract: We included five studies enrolling adults with frequent episodic TTH; 1812 participants took medication, of which 767 were included in comparisons of aspirin 1000 mg with placebo, and 405 in comparisons of aspirin 500 mg or 650 mg with placebo. Not all of these participants provided data for outcomes of interest in this review. Four studies specified using IHS diagnostic criteria; one predated commonly recognised criteria, but described comparable characteristics and excluded migraine. All participants treated headaches of at least moderate pain intensity. None of the included studies were at low risk of bias across all domains considered, although for most studies and domains this was likely to be due to inadequate reporting rather than poor methods. We judged one study to be at high risk of bias due to small size. There were no data for aspirin at any dose for the IHS preferred outcome of being pain free at two hours, or for being pain free at any other time, and only one study provided data equivalent to having no or mild pain at two hours (very low quality evidence). Use of rescue medication was lower with aspirin 1000 mg than with placebo (2 studies, 397 participants); 14% of participants used rescue medication with aspirin 1000 mg compared with 31% with placebo (NNTp 6.0, 95% confidence interval (CI) 4.1 to 12) (low quality evidence). Two studies (397 participants) reported a Patient Global Evaluation at the end of the study; we combined the top two categories for both studies to determine the number of participants who were 'satisfied' with treatment. Aspirin 1000 mg produced more satisfied participants (55%) than did placebo (37%) (NNT 5.7, 95% CI 3.7 to 12) (very low quality evidence). Adverse events were not different between aspirin 1000 mg and placebo (RR 1.1, 95% CI 0.8 to 1.5), or aspirin 500 mg or 650 mg and placebo (RR 1.3, 95% CI 0.8 to 2.0) (low quality evidence). Studies reported no serious adverse events. The quality of the evidence using GRADE comparing aspirin doses between 500 mg and 1000 mg with placebo was low or very low. Evidence was downgraded because of the small number of studies and events, and because the most important measures of efficacy were not reported. There were insufficient data to compare aspirin with any active comparator (paracetamol alone, paracetamol plus codeine, peppermint oil, or metamizole) at any of the doses tested. A single dose of aspirin between 500 mg and 1000 mg provided some benefit in terms of less frequent use of rescue medication and more participants satisfied with treatment compared with placebo in adults with frequent episodic TTH who have an acute headache of moderate or severe intensity. There was no difference between a single dose of aspirin and placebo for the number of people experiencing adverse events. The amount and quality of the evidence was very limited and should be interpreted with caution.

Plain-language summary: In September 2016, we searched the medical literature and found five studies involving 1812 participants looking at aspirin for frequent episodic tension-type headache. About 1668 participants were involved in comparisons between aspirin at doses between 500 mg and 1000 mg and placebo (a dummy tablet). The International Headache Society recommends the outcome of being pain free two hours after taking a medicine, but other outcomes are also suggested. No studies reported pain free at two hours, or other recognised outcomes, so there was limited information to analyse for outcomes about how well aspirin works. None of the studies reported on participants being pain free at two hours, and only one study reported an outcome we judged equivalent to being pain free or having only mild pain at two hours. For aspirin 1000 mg, about 10 participants in 100 used additional painkillers, compared with 30 in 100 with placebo (very low quality evidence). At the end of the study 55 in 100 participants were 'satisfied' with treatment compared with 37 in 100 with placebo (very low quality evidence). About 15 in 100 people taking aspirin 1000 mg reported having a side effect after one dose, which was the same as with placebo (low quality evidence). The quality of the evidence was low or very low for the comparisons between aspirin and placebo. Low and very low quality evidence means that we are very uncertain about the results.