Aspirin: how low is low dose?

Aspirin reduces the risk of non-fatal stroke, non-fatal myocardial infarction and vascular death in patients at high risk of arterial thrombosis. Platelet function is inhibited by doses of 40-160 mg daily. In clinical trials performed before 1985, doses of 500-1500 mg daily were found to be effective. Since 1985, low doses have been evaluated and a meta-analysis has shown that daily doses of 75-150 mg are as effective as the previously used higher doses. The risk of adverse effects is dose-dependent above 75 mg daily. The maximum benefit:risk ratio is likely to be achieved with doses of 75-150 mg daily. In Australia, this is best achieved by either a single 100 mg tablet or half a 300 mg tablet taken daily.

For almost 100 years, aspirin (acetylsalicylic acid) has been used extensively for its effective analgesic, antipyretic and anti-inflammatory properties. In the last 25 years, since its ability to inhibit platelet function was discovered, aspirin has been increasingly used as an antithrombotic agent. There are now extensive data from clinical trials supporting its effectiveness in the prevention of thrombosis.1 The analyses of the Antiplatelet Trialists' Collaboration2 show conclusively that antiplatelet therapy (principally aspirin) reduces the risk of non-fatal stroke, non-fatal myocardial infarction and vascular death by 25-32% in patients at high risk of arterial thrombosis. In addition, there are significant reductions in the incidence of occlusion of grafts and native vessels after vascular surgery and of venous thromboembolism in high-risk patients.

In clinical trials before 1985, the doses of aspirin used (500 -1500 mg daily in divided doses) were considerably greater than required for maximum inhibition of platelet function. More recent evidence has shown that lower doses (75-325 mg daily as a single dose) are clinically effective. Furthermore, dose-ranging studies with different formulations show that the maximum effect of aspirin on platelet function is attained by doses of 40-160 mg daily.³

In platelets, the predominant product of the pathway for synthesis of prostaglandins is thromboxane A2 which is a powerful promoter of platelet aggregation. In a developing thrombus, thromboxane is produced by stimulated platelets and secreted into the surrounding medium. There it acts synergistically with other platelet stimuli to enhance platelet stickiness and, hence, platelet aggregation. Aspirin prevents the production of thromboxane by inhibiting the enzyme cyclooxygenase. This leads to inhibition of the mechanisms of both haemostasis and thrombosis, as shown by a prolongation of the bleeding time and by a decreased tendency to arterial thrombosis in experimental animals. As aspirin binds to platelet cyclooxygenase irreversibly, it inhibits the function of the platelets for the rest of their 8-10 day lifespan.

Aspirin also inhibits cyclooxygenase in the endothelium of the arteries and veins, and hence blocks the production of prostacyclin, a powerful inhibitor of platelet aggregation. This aspirin-induced loss of prostacyclin production potentially reduces the overall antithrombotic action of aspirin, but the clinical significance is not known. The inhibition of prostacyclin formation is reversible, because the endothelium is capable of resynthesising cyclooxygenase.

Lower doses

It seems reasonable to use doses of aspirin which, while lower than those used previously, are still sufficient to exert a maximum inhibitory effect on platelet function. The two main reasons for this are to minimise adverse effects, and to attempt to spare prostacyclin production in the vessel wall.

Adverse effects

The adverse effects of aspirin are mainly gastrointestinal.⁴ They are dose related so they are reduced at lower doses (325 mg per day or less). Aspirin may cause gastric erosions and gastric ulcers and patients may present with anaemia and/or haemorrhage. Longitudinal studies show that 75 mg daily causes a small but significant increase in gastrointestinal bleeding, and this effect doubles with 300 mg daily and increases 5-fold with 1.8-2.4 g daily.⁴

An important, but uncommon, adverse effect is haemorrhagic stroke. The incidence was estimated to be 0.7 per 1000 patients treated in a subset of trials where the reduction in ischaemic stroke was 10 per 1000 patients treated.⁵ However, unlike the risk of gastrointestinal bleeding, the risk of haemorrhagic stroke would not be expected to decrease unless very low doses of aspirin were used, doses which do not maximally inhibit platelet function. Currently, there is little or no evidence to indicate that such very low doses would be adequate to prevent thrombosis.

Sparing of prostacyclin production

A further advantage of lower doses of aspirin might be that inhibition of prostacyclin formation in the vessel wall could be minimised. Preservation of prostacyclin production may be valuable in the prevention of thrombosis. Many dosage regimens have been investigated to find one which leads to maximal inhibition of platelet thromboxane formation while sparing inhibition of prostacyclin formation. Most recent studies suggest that any sparing of prostacyclin production is likely to be minimal, and the resultant clinical benefit will be small and therefore difficult to demonstrate in a clinical trial.¹ Hence, the arguments for use of lower doses of aspirin continue to rest on the lesser frequency and severity of adverse effects.

Different formulations

Adverse effects

On platelet function

Studies in normal volunteers have revealed much of the relationship between the dose of aspirin and the degree of inhibition of platelet function.³ It is important to distinguish between the effects of a single dose of aspirin and the effects of repeated daily doses. When given as a single dose, 325 mg of soluble aspirin is sufficient to maximally inhibit platelet function within 30 minutes. Hence, this dose of aspirin as a solution or rapid release formulation should be given as a loading dose in situations, such as myocardial infarction, where an immediate effect is desirable.³ When given as a daily dose of 40-80 mg, soluble aspirin has a cumulative effect such that it maximally inhibits platelet thromboxane formation by >95% after 4-5 days. When given as a daily dose of 80-100mg, an enteric-coated preparation also produces cumulative and near complete inhibition of agonist-induced platelet aggregation and thromboxane formation in 3-5 days.

In clinical trials

The evidence from clinical trials shows that all doses which maximally inhibit platelet function are effective in the prevention of thromboembolism. The Antiplatelet Trialists' Collaboration2 has published a meta-analysis of 145 randomised clinical trials of antiplatelet therapy. Forty-six trials (some trials contributed to more than one comparison) in which aspirin alone was used were analysed to compare 3 dosage regimens:

• <160 mg (mostly 75-150 mg) (7 trials)
• 160-325 mg (12 trials)
• 500-1500 mg daily (30 trials)

Risk reductions for myocardial infarction, stroke or vascular death were not significantly different for these 3 regimens, being 26%, 28% and 21% respectively. Three trials were analysed in which a higher dose of aspirin (500-1500 mg daily) was compared with a lower dose (75-325 mg daily). There was no significant difference in efficacy between these regimens. These trials provide substantial evidence that doses as low as 75 mg daily are effective, but no substantial evidence about daily doses less than 75 mg.

Soluble aspirin in doses of 75-100 mg daily almost completely inhibits cyclooxygenase in both platelets and arterial walls. Whether lower doses have a greater or lesser therapeutic effect is unknown. Neither is it known whether slow release formulations at doses which are just sufficient to substantially inhibit platelet function might significantly spare prostacyclin production to provide a probably small, but possibly significant, clinical advantage.¹

There remains some controversy concerning the application of the above conclusions to the prevention of ischaemic stroke. It has been suggested that, for patients with a prior stroke or transient ischaemic attack, doses as high as 1000 mg per day may be preferable⁷, although the clinical trials provide little evidence to support this.²

In Australia, aspirin is available as 100 mg tablets (3 preparations, two of which are enteric-coated). The next highest dose available is 300 mg (at least 5 preparations). Hence, doses of 75-150 mg can be obtained by daily ingestion of either a single 100 mg tablet or half a 300 mg tablet. Alternatively, 150 mg taken on alternate days would seem reasonable, but there are no clinical trial data to support such a regimen.

Low doses of aspirin of the order of 75-300 mg daily are as effective as higher doses for the prevention of arterial thrombosis. Although in many of the clinical trials higher doses were used, there is now reasonable evidence that lower doses are probably effective. To reduce the incidence of adverse effects, it seems reasonable to prescribe a low dose. Where cost is not a limiting factor, an enteric-coated or slow release preparation can be used, which may reduce the incidence of gastrointestinal blood loss and indigestion. Many patients will prefer the least expensive option which is about 150 mg of ordinary or soluble aspirin (half a standard 300 or 325 mg tablet).

The following statements are either true or false.

1. The lowest dose of aspirin recommended for the prevention of arterial thrombosis is 75 mg.

2. Inhibition of endothelial cyclooxygenase by aspirin potentially increases platelet aggregation.