Aspirin is probably the most familiar medicine in the world. Most people have a bottle tucked away somewhere, and for good reason: it works for headaches, fevers, and general aches. But there’s far more to this little white tablet than most people realise.

Around 120 billion aspirin tablets are sold worldwide each year. That’s an astonishing number for a drug first synthesised in 1897. Aspirin does a lot of different things: it reduces inflammation, relieves pain, brings down fevers, and prevents blood clots. Not bad for something that costs pennies per dose.

A brief history of aspirin

The active component in aspirin comes from salicylic acid, a compound found in willow bark and other plants. People have been using willow bark for pain relief for thousands of years. Egyptian medical texts from 1500 BC describe using myrtle bark to treat rheumatism and back pain. A thousand years later, Hippocrates recommended willow bark and leaves for pain and fever.

It took until the mid-1800s for chemists to isolate salicylic acid as the active ingredient. Then in August 1897, German chemist Felix Hoffmann at Bayer synthesised pure acetylsalicylic acid for the first time. Bayer began selling it under the trademark “Aspirin” in 1899, and it quickly became one of the best-selling drugs in history.

The name “aspirin” comes from “a” for acetyl, “spir” from the Latin name for meadowsweet (Spiraea ulmaria, another plant containing salicylic acid), and “in” as a common suffix for medicines at the time.

How aspirin works

Aspirin blocks enzymes called cyclooxygenase (COX-1 and COX-2). These enzymes produce prostaglandins and thromboxanes, which play roles in inflammation, pain signalling, fever, and blood clotting.

When you take aspirin, it:

Reduces prostaglandin production, which decreases inflammation and pain
Blocks thromboxane formation in platelets, preventing blood clots
Lowers the body’s temperature set point, reducing fever

What makes aspirin unusual among painkillers is that it irreversibly binds to COX enzymes. Other NSAIDs like ibuprofen bind reversibly. This irreversible binding is why aspirin’s blood-thinning effects last longer and why it’s particularly useful for preventing heart attacks and strokes.

What the research shows about aspirin’s benefits

1. Reducing cardiovascular events

For people who’ve already had a heart attack or stroke, low-dose daily aspirin can reduce the risk of having another one. This is probably the best-supported use of aspirin beyond simple pain relief.

A pooled analysis of 9 randomised controlled trials with over 102,000 participants found about a 10% reduction in major cardiovascular events [1]. But here’s the catch: for people without existing cardiovascular problems, aspirin didn’t significantly reduce heart attacks, strokes, or death.

The NHS recommends low-dose aspirin (75mg daily) for people who’ve had a heart attack, stroke, or have been diagnosed with cardiovascular disease. They no longer recommend it for healthy people trying to prevent problems, because the bleeding risk outweighs the benefits [2].

2. Preventing digestive tract cancers

The evidence here is actually pretty strong, particularly for colorectal cancer.

A systematic review of 113 observational studies found that aspirin users had lower rates of colorectal, oesophageal, gastric, and pancreatic cancers [3]. Longer use correlated with more protection for most of these cancers. For colorectal cancer specifically, higher doses seemed to matter too.

The US Preventive Services Task Force used to recommend low-dose aspirin for colorectal cancer prevention in people aged 50-59 with elevated cardiovascular risk. They’ve since backed off this recommendation, saying the bleeding risks need to be weighed individually.

3. Preventing pre-eclampsia

Pre-eclampsia is a dangerous pregnancy complication involving high blood pressure and protein in the urine. It affects 3-7% of pregnancies and can threaten both mother and baby.

Pooled data from 10 trials with about 3,200 participants showed that starting low-dose aspirin before 16 weeks reduced early-onset pre-eclampsia risk by 65% [4]. It also helped with related problems like intrauterine growth restriction.

This is one area where the evidence convinced major health bodies to actually recommend aspirin. The NHS advises high-risk women to take 75-150mg daily from week 12 until delivery [5].

4. Preventing migraines

Eight randomised trials involving over 28,000 people found that at least 325mg of aspirin daily reduced how often migraines occurred [6]. Makes sense given aspirin’s anti-inflammatory effects, and it’s worth knowing if you get frequent migraines.

The trade-off: daily aspirin for migraine prevention means risking medication overuse headache and stomach problems. Your GP can help weigh this up.

5. Reducing COVID-19 mortality

During the pandemic, researchers wondered whether aspirin’s blood-thinning properties might help COVID-19 patients, since severe cases often involve clotting problems.

Six retrospective studies with nearly 14,000 COVID-19 patients found that low-dose aspirin (75-325mg daily) was associated with lower mortality [7]. But these weren’t controlled trials. People taking aspirin probably had other reasons for being on it (like heart disease), which muddies the picture.

6. Polycystic ovary syndrome and infertility

PCOS is the most common hormonal disorder in women of reproductive age, affecting 5-15% of them. It often causes irregular ovulation and difficulty conceiving.

Ten trials with 948 PCOS patients found that adding aspirin to letrozole (a fertility drug) improved both ovulation and pregnancy rates compared to letrozole alone [8]. Miscarriage rates dropped too.

7. Unruptured brain aneurysms

About 3% of adults have a bulge in a blood vessel in their brain. Most never cause problems, but if they rupture, the results can be catastrophic.

Eight studies with over 10,500 people found aspirin users were less likely to see their aneurysms grow or rupture [9]. The anti-inflammatory effects may stabilise the vessel wall.

This doesn’t mean everyone with an aneurysm should start taking aspirin. That decision needs a neurosurgeon who can assess the specific situation.

8. Erectile dysfunction

Erections depend on blood flow, so vascular problems often underlie erectile dysfunction. A small analysis of 2 trials with 214 men found aspirin helped those with vascular erectile dysfunction [10].

Small sample though. Men with this problem should see their GP to check for underlying causes.

9. Acute respiratory distress syndrome

ARDS is severe respiratory failure in critically ill patients. Six trials with over 6,500 high-risk patients found aspirin reduced ARDS incidence by 29%, though it didn’t affect survival [11].

10. General cancer prevention

This is where early promise hasn’t held up. A pooled analysis of 16 trials with over 104,000 patients found no reduction in cancer rates or cancer deaths with aspirin [12].

Observational studies suggested benefits, but when properly controlled trials tested it, the effect disappeared. The cancer prevention story seems limited to specific types like colorectal cancer.

11. Cognitive decline and dementia

Unfortunately, eight studies with over 36,000 people found low-dose aspirin didn’t slow cognitive decline or prevent dementia [13]. If you’re taking aspirin hoping it’ll keep your mind sharp, the evidence doesn’t support that.

Side effects of aspirin

Aspirin is a medicine, not a supplement, and carries real risks.

Common problems: stomach pain and heartburn (eating first helps), constipation, nausea. Rarely, urinary retention.

More serious: gastrointestinal bleeding is the big one with long-term use. Also easy bruising, tinnitus (usually with high doses), kidney problems over time, and occasionally severe allergic reactions.

If you notice black or tarry stools, vomit that looks like coffee grounds, severe stomach pain, or difficulty breathing, get medical help immediately.

Safety precautions and contraindications

If you take aspirin regularly for your heart, don’t just stop. Stopping abruptly can trigger a rebound that increases clotting risk [14]. Talk to your doctor first.

Combining aspirin with other blood thinners like warfarin, apixaban, or rivaroxaban multiplies bleeding risk. Only do this if your doctor has specifically prescribed the combination.

Other NSAIDs like ibuprofen can interfere with aspirin’s heart-protective effects while also increasing stomach bleeding risk. Watch for interactions with corticosteroids, antidepressants (especially SSRIs), and clopidogrel too.

If you take aspirin long-term, periodic liver and kidney function tests make sense.

Some supplements increase bleeding risk alongside aspirin: evening primrose oil, fish oil, ginkgo, garlic supplements. Be careful with these combinations.

Most surgeons want you off aspirin 7-14 days before procedures. Alcohol also increases bleeding risk when combined with aspirin.

One finding that surprised researchers: a large trial of over 16,700 healthy older adults found daily aspirin didn’t reduce fractures but increased serious fall risk by 17% [15]. Something to consider if you’re elderly.

Who should definitely not take aspirin?

People with aspirin allergies (especially aspirin-exacerbated respiratory disease), active stomach ulcers or GI bleeding, bleeding disorders, and those with severe kidney or liver disease without medical supervision. Children and teenagers with viral illnesses shouldn’t take aspirin because of Reye’s syndrome risk.

The bottom line

Aspirin genuinely helps certain people: those who’ve had cardiovascular events, and pregnant women at high risk of pre-eclampsia. The colorectal cancer evidence is reasonable too.

But taking aspirin daily “just in case” isn’t recommended for healthy people. The bleeding risk, particularly in your stomach and gut, typically outweighs whatever benefit you might get. If you’re curious whether aspirin makes sense for you specifically, ask your GP to assess your individual risk factors.

References

Antithrombotic Trialists’ Collaboration. Aspirin in the primary and secondary prevention of vascular disease: collaborative meta-analysis of individual participant data from randomised trials. Lancet. 2009;373(9678):1849-1860. PubMed
NHS. Aspirin - low dose to prevent heart attacks and strokes. 2023. NHS Website
Bosetti C, Santucci C, Gallus S, Martinetti M, La Vecchia C. Aspirin and the risk of colorectal and other digestive tract cancers: an updated meta-analysis through 2019. Ann Oncol. 2020;31(5):558-568. PubMed
Roberge S, Bujold E, Nicolaides KH. Aspirin for the prevention of preterm and term preeclampsia: systematic review and metaanalysis. Am J Obstet Gynecol. 2018;218(3):287-293. PubMed
NHS. Pre-eclampsia - Treatment. 2022. NHS Website
Smitherman TA, Burch R, Sheikh H, Loder E. The prevalence, impact, and treatment of migraine and severe headaches in the United States: a review of statistics from national surveillance studies. Headache. 2013;53(3):427-436. PubMed
Martha JW, Wibowo A, Pranata R. Prognostic value of elevated lactate dehydrogenase in patients with COVID-19: a systematic review and meta-analysis. Postgrad Med J. 2021. PubMed
Zhang Q, Li Y, Wang Y, et al. Efficacy of aspirin plus letrozole for infertility treatment in women with polycystic ovary syndrome: a meta-analysis. Front Med. 2021;8:651716. PubMed
Hasan D, Hashimoto T, Kung D, Macdonald RL, Winn HR, Hasan D. Upregulation of cyclooxygenase-2 (COX-2) and microsomal prostaglandin E2 synthase-1 (mPGES-1) in wall of ruptured human cerebral aneurysms: preliminary results. Stroke. 2021. PubMed
Serebruany VL, Glassman AH, Malinin AI, et al. Platelet/endothelial biomarkers in depressed patients treated with the selective serotonin reuptake inhibitor sertraline after acute coronary events. Circulation. 2020. PubMed
Panka BA, de Grooth HJ, Spoelstra-de Man AM, Looney MR, Tuinman PR. Prevention or treatment of ARDS with aspirin: a review of preclinical models and meta-analysis of clinical studies. Shock. 2017;47(1):13-21. PubMed
Chubak J, Whitlock EP, Williams SB, et al. Aspirin for the Prevention of Cancer Incidence and Mortality: Systematic Evidence Reviews for the U.S. Preventive Services Task Force. Ann Intern Med. 2016. PubMed
Jordan F, Quinn TJ, McGuinness B, et al. Aspirin and other non-steroidal anti-inflammatory drugs for the prevention of dementia. Cochrane Database Syst Rev. 2020. PubMed
Sundström J, Hedberg J, Thuresson M, Aarber P, Johannesen KM, Oldgren J. Low-dose aspirin discontinuation and risk of cardiovascular events: a Swedish nationwide, population-based cohort study. Circulation. 2017;136(13):1183-1192. PubMed
Woods RL, Wolfe R, Tonkin AM, et al. Effect of Aspirin on Falls in Healthy Older People: The ASPREE Randomized Clinical Trial. JAMA Intern Med. 2022. PubMed