Undesirable Side Effects
Aspirin, in whatever formulation, and even at low doses, can cause gastric irritation, increased occult blood loss and occasionally, serious gastric bleeding. (51)

These effects are strongly dose related. Evidence from the published trials suggests that although the relative risk of gastric irritation is increased by 50 to 100%, the absolute risk is only about 5% above the proportion in subjects given a placebo tablet. That is, about one in every twenty subjects on a low-dose regime is likely to be affected. Occult bleeding is on average about doubled by aspirin, though clinically significant blood loss is likely to occur in only about two or three in every thousand subjects.

The question of an increased risk of cerebral haemorrhage is unresolved. Data from a number of trials have suggested that the risk may be increased by aspirin, yet when evidence from all the trials is put together in overviews, the excess risk is not statistically significant. It seems reasonable to assume therefore that the increase in risk of cerebral haemorrhage is, at most, extremely small.

Overall balance in aspirin prophylaxis
(based on various reports)
A reduction of 30% in relative risk means that of every 1000 subjects judged suitable for low-dose aspirin...

about 35 who have had a previous MI, stroke or TIA will avoid a vascular event (MI, stroke or TIA) per year

• about 12 who have stable angina will avoid an event each year
  
• about the same number who are judged to be at “high risk” for other reasons will avoid an event each year

and

• perhaps one or two will suffer a major bleed
  
• perhaps three or four will experience a minor bleed
  
• perhaps 50 more will have gastric irritation severe enough to withdraw from prophylaxis