One out of five people suffer from tension-type headache around the world. It is mainly divided as: infrequent episodic TTH (fewer than one headache per month), frequent episodic TTH (two to 14 headache days per month), and chronic TTH (15 headache days per month or more). For acute  episodic TTH treatment, aspirin is recommended as one of the number of analgesics. The efficacy and safety of aspirin for acute treatment of episodic tension-type headache (TTH) in adults compared with placebo or any active comparator has been investigated in this study.

The Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, Embase, and the Oxford Pain Relief Database were explored from initiation to September 2016 along with the  reference lists of supporting published studies and reviews. The unpublished studies were soughted by asking personal contacts and exploring online clinical trial registers and manufacturers' websites.

Randomised, double-blind, placebo-controlled studies (parallel-group or cross-over) were inculcated using oral aspirin for symptomatic relief of an acute episode of TTH. These studies considered participants aged 18 years or over, and include at least 10 participants per treatment arm.

The studies for inclusion and extracted data were independently examined by two review authors. The risk ratio (RR) and number required to treat for one additional beneficial outcome (NNT), one additional harmful outcome (NNH), or to prevent one event (NNTp) for oral aspirin compared to placebo or an active intervention were noted for different outcomes (specifically those advocated by the International Headache Society (IHS)). GRADE was used to assess the evidence and a 'Summary of findings' table was made.

Five studies were considered comprising of adults with frequent episodic TTH. A total of 1812 participants took medication, of which 767 were considered in comparisons of aspirin 1000 mg with placebo, and 405 in comparisons of aspirin 500 mg or 650 mg with placebo. In this review, not all supplemented the outcomes for the relevant data. Four studies were identified via the IHS diagnostic criteria. Out of these four, one predated commonly recognised criteria, but reported comparable characteristics and excluded migraine. The headaches treated were of least moderate pain intensity.

None of the included studies were at low risk of bias across all domains considered, while for most studies and domains this was likely because of inadequate reporting rather than poor techniques. Due to small size, one study was believed to be at high risk of bias.

At two hours, there was no data for aspirin at any dose for the IHS preferred outcome of being pain free or for that matter being pain free at any other time, and only one study supplemented data equivalent to having no or mild pain at two hours (very low quality evidence). The application 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 as distinguished from 31% with placebo (NNTp 6.0, 95% confidence interval (CI) 4.1 to 12) (low quality evidence).

At the closure of the study, two studies delineated a Patient Global Evaluation when the top two categories were merged for both studies to reveal the number of participants who were 'satisfied' with the treatment. More satisfied participants (55%) were evaluated for aspirin 1000 mg than did placebo (37%) (NNT 5.7, 95% CI 3.7 to 12) (very low quality evidence). 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) revealed no different adverse events.  No serious adverse events were reported in these studies.

Low quality evidence was found using GRADE comparing aspirin doses between 500 mg and 1000 mg with placebo. Due to small number small number of studies and events and lack of reports regarding significant efficacy measures, the evidence was degraded. At any experimental doses, there was insufficient data to compare aspirin with any active comparator (paracetamol alone, paracetamol plus codeine, peppermint oil, or metamizole).

It was culminated that a single dose of aspirin between 500 mg and 1000 mg provided some advantage in terms of less frequent use of rescue medication. More patient satisfaction was observed with treatment compared with placebo in adults with frequent episodic TTH who have an acute headache of moderate or severe intensity. For the number of people undergoing adverse events, no difference between a single dose of aspirin and placebo was observed. Limited amount and quality of the evidence was available and it should be elucidated with caution.