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Acetylsalicylic acid — better known as ASA or aspirin — is one of the oldest and most versatile drugs in the world. In low doses (100 mg), it protects millions of cardiovascular patients from heart attacks and strokes. In higher doses (500—1,000 mg), it alleviates pain, fever and inflammation.
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This article is for information only and does not replace medical advice. Low-dose aspirin (100 mg) for blood thinning is prescription only for long-term use and should never be started or stopped without consulting a doctor.
Acetylsalicylic acid (ASA), better known as aspirin, is one of the oldest and best-selling active substances in the world – and at the same time one of the most frequently misunderstood. Depending on the dose it acts as a painkiller, an antipyretic, or a blood thinner. The overview below shows the most important pharmacological key facts.
| Property | Details |
|---|---|
| Active substance | Acetylsalicylic acid (ASA) |
| ATC code | B01AC06 (antiplatelet agent) / N02BA01 (analgesic) |
| Drug class | NSAID / antiplatelet agent |
| Available forms | Tablets (100 mg, 300 mg, 500 mg), effervescent tablets, chewable tablets, granules, IV solution |
| Half-life | ASA: approx. 15–20 min (rapidly converted to salicylic acid); salicylic acid: 2–3 hours |
| Duration of effect on platelets | 7–11 days (lifespan of the platelets) – irreversible! |
| Prescription status | Over the counter as a painkiller; low-dose aspirin for long-term use is prescribed by a doctor |
| Special feature | The only NSAID with irreversible platelet inhibition |
What many people do not know: aspirin is not one medication but two – depending on the dose. At a high dose (500–1,000 mg) it acts as a painkiller and antipyretic. At a low dose (75–100 mg daily) it is a heart-protective medication that prevents platelets from clumping together. Both uses have different target groups, risks, and rules of use. Anyone who knows both avoids dangerous mistakes.
| Aspirin as a painkiller | Aspirin for blood thinning | |
|---|---|---|
| Dose | 500–1,000 mg per intake | 75–100 mg per day |
| Intake | As needed (max. 3,000 mg/day) | Daily as long-term medication |
| Effect | Pain relief, fever reduction, anti-inflammatory action | Platelet aggregation inhibition |
| Target group | Headaches, migraine, toothache, fever | After heart attack, stroke, stent, CHD, PAD |
| Stomach risk | High with frequent intake | Increased, hence often with a PPI (pantoprazole) for stomach protection |
The most common mistake in practice: patients who take low-dose aspirin for blood thinning reach for another aspirin tablet at a higher dose when they have a headache – or for ibuprofen. Both can jeopardise the heart protection or massively increase the bleeding risk. More on this in chapter 6.
Aspirin inhibits the enzyme cyclo-oxygenase (COX) – similarly to ibuprofen and diclofenac. The decisive difference: aspirin inhibits COX irreversibly through acetylation. The enzyme is switched off permanently and cannot be repaired. This is the reason why a single missed dose of low-dose aspirin has hardly any consequences – but also why the effect only wears off completely 7–11 days after stopping.
At a low dose, aspirin mainly inhibits COX-1 in the platelets. This produces less thromboxane A2 – a substance that normally makes platelets clump together. As platelets have no cell nucleus, they cannot remanufacture the destroyed enzyme. The effect therefore lasts as long as the platelet lives: 7–11 days.
At a higher dose, aspirin additionally inhibits COX-2 in various tissues. This reduces prostaglandins, which are responsible for pain, fever, and inflammation – but at the same time also maintain the protective stomach lining. This explains the side effects on the stomach.
The right dose of aspirin depends entirely on the treatment goal. Confusing the dose levels can be dangerous – doses that are too high increase the bleeding risk, doses that are too low do not work sufficiently for pain. Record your aspirin dose in your digital medication plan.
| Indication | Dose | Intake | Max. duration without a doctor |
|---|---|---|---|
| Heart protection / blood thinning | 75–100 mg/day | 1× daily, ongoing | Only on prescription |
| After a stent (dual antiplatelet therapy) | 100 mg/day + clopidogrel/ticagrelor | Daily, 6–12 months | Strictly as directed by a doctor |
| Acute heart attack (first measure) | 250–500 mg | Chew once (!) | Emergency |
| Pain / fever (adults) | 500–1,000 mg | Every 4–6h, max. 3,000 mg/day | Max. 3–4 days |
| Migraine (guideline recommendation) | 1,000 mg | Once during an attack | – |
The main risk of aspirin lies in the gastrointestinal tract and in an increased tendency to bleed. Both risks also exist at the low 100 mg dose and should not be underestimated – particularly in patients over 65 or when taking other blood-thinning medications at the same time. Anyone who systematically documents side effects can respond early.
Aspirin inhibits the formation of protective prostaglandins in the stomach lining. This increases the risk of stomach ulcers and gastrointestinal bleeding. Enteric-coated tablets (e.g. Aspirin protect) reduce local stomach irritation but do not prevent the systemic effect on the stomach lining. In high-risk patients, a PPI such as pantoprazole or omeprazole is sensible as stomach protection.
| Side effect | Frequency | What to do? |
|---|---|---|
| Stomach pain / heartburn | Common | Always take after eating. A PPI for stomach protection if needed |
| Gastrointestinal bleeding | Occasional | Black stools = emergency! See a doctor at once |
| Increased tendency to bleed | Common | Bruises more easily, longer bleeding from injuries |
| Nosebleeds, bleeding gums | Common | Inform your doctor if it occurs frequently |
| Analgesic asthma (aspirin intolerance) | Rare (but: 10–20% of people with asthma!) | Aspirin contraindicated! Alternative: clopidogrel |
| Allergic reactions | Rare | Shortness of breath, skin rash → emergency services (112; or 999/112 in the UK) |
| Reye's syndrome (children!) | Very rare, potentially fatal | Aspirin contraindicated in children under 16 (exception: Kawasaki) |
Particularly important: black stools are a warning sign of gastrointestinal bleeding and a medical emergency. Anyone on long-term aspirin therapy who notices unusual abdominal pain or changes in their stools should see a doctor at once.
This is the most dangerous and at the same time most frequently overlooked interaction in patients on long-term aspirin therapy. Many people reflexively reach for ibuprofen when they have back pain, headaches, or joint pain – without knowing that they are jeopardising their heart protection. The interaction check from brite detects this combination automatically.
| Scenario | Blood thinning preserved? | Recommendation |
|---|---|---|
| Low-dose aspirin in the morning, ibuprofen 4+ h later | Yes – aspirin has already bound irreversibly | A possible solution for occasional need |
| Ibuprofen first, low-dose aspirin later | No – ibuprofen blocks the binding site | Dangerous: heart protection cancelled out! |
| Ibuprofen and aspirin at the same time | No – ibuprofen "wins" the race | Avoid! |
| Paracetamol instead of ibuprofen | Yes – does not affect the aspirin effect | Best alternative on long-term aspirin |
Besides the ibuprofen trap there are other clinically relevant interactions – especially for patients on long-term medication with blood thinners or stomach medicines. Check your combination with the interaction check.
| Substance / medication | Interaction | Recommendation |
|---|---|---|
| Marcumar / phenprocoumon | Massively increased bleeding risk | Only under strict medical supervision, monitor INR closely |
| DOACs (apixaban, rivaroxaban) | Increased bleeding risk | Avoid triple therapy, stop aspirin as early as possible after a stent |
| Corticosteroids (prednisolone) | Strongly increased risk of stomach bleeding | Always add a PPI (stomach protection) |
| Methotrexate | Aspirin reduces methotrexate excretion → toxicity | Avoid the combination or monitor closely |
| SSRIs (citalopram, sertraline) | SSRIs increase the bleeding risk further | PPI stomach protection recommended |
| Alcohol | Strongly increased risk of stomach bleeding | Minimise alcohol on long-term aspirin |
| Ramipril / ACE inhibitors | Aspirin can slightly weaken the effect of ACE inhibitors | Usually not clinically relevant at low-dose aspirin |
Especially for patients on blood-thinning therapy: the combination of aspirin + Marcumar or aspirin + a DOAC requires close medical monitoring. The bleeding risk adds up – and small injuries can lead to serious complications. Use the medication list to keep track of all combinations.
