Whew, lots of questions.
Does aspirin reduce risk of heart-disease or stroke?
The Antithrombotic Trialists' Collaboration published Collaborative meta-analysis of randomised trials of antiplatelet therapy for prevention of death, myocardial infarction, and stroke in high risk patients. BMJ. 2002 Jan 12;324(7329):71-86.
Aspirin (or another oral antiplatelet drug) is protective in most types of patient at increased risk of occlusive vascular events, including those with an acute myocardial infarction or ischaemic stroke, unstable or stable angina, previous myocardial infarction, stroke or cerebral ischaemia, peripheral arterial disease, or atrial fibrillation. Low dose aspirin (75-150 mg daily) is an effective antiplatelet regimen for long term use, but in acute settings an initial loading dose of at least 150 mg aspirin may be required.
They recommend aspirin (or equivalent) for known high risk people. They don't know if it is possible to generally detect high risk people from the healthy population. They don't recommend aspirin for everyone.
These results reinforce the value of ensuring that antiplatelet therapy with 75-150 mg aspirin daily (or some other effective antiplatelet regimen) is considered routinely for all such patients at high or intermediate risk of occlusive vascular events (more than about 2% a year), irrespective of whether they have already had a major vascular event. An unanswered question, however, is whether it is possible to identify particular groups of apparently healthy people who may be at increased risk of myocardial infarction or stroke and for whom the benefits of daily aspirin outweigh the hazards. This is currently being investigated in an analysis of primary prevention trials. For most healthy individuals, however, for whom the risk of a vascular event is likely to be substantially less than 1% a year, daily aspirin may well be inappropriate.
(Note: An errata has been applied to this section.)
Does aspirin reduce risk of cancers?
Kind of. Maybe. No. Cancer is a big area; it isn't a single disease.
Aspirin reduces the chance of some cancers from recurring. For example:
There was evidence from three pooled [Randomly Controlled Trials] that [Aspirin] significantly reduces the recurrence of sporadic adenomatous polyps after one to three years. There is evidence from short-term studies to support regression, but not elimination or prevention of [colorectal adenomas] in [familial adenomatous polyposis].
Wikipedia has a reasonable discussion on some of the still-too-early-to-tell results, which includes:
- Not helpful for prostate.
- Mixed results for pancreatic.
- Some evidence for colon, and lung.
- Inconclusive for upper GI tract.
Does aspirin reduce risk of Alzheimer's disease?
The jury is still out.
This paper found that aspirin may help:
As expected, there was an inverse association between nonsteroidal anti-inflammatory drugs and aspirin (and unexpectedly angiotensin-converting enzyme inhibitors) and AD. This association was not observed with vascular dementia or any other diagnoses. Analysis showed no evidence for a dosage effect, ie, responses were equivalent for low and high doses.
Whereas this one suggests it may harm:
This paper concluded it didn't help existing Alzheimer's patients:
Although aspirin is commonly used in dementia, in patients with typical [Alzheimer's Disease] 2 years of treatment with low-dose aspirin has no worthwhile benefit and increases the risk of serious bleeds.
Conflicting studies? Time to look for a meta-analysis. The Cochrane Collaboration don't know yet, but they plan to find out.
Does aspirin have any adverse side-effects?
Yes. Many, many side-effects - generally mild or rare.
This (unreferenced, but likely reliable) pharmaceuticals site lists many of them. Here are some reduced highlights:
Gastrointestinal, including gastric mucosal lesions (very common), epigastric distress (very common), gastrointestinal bleeding (common in the elderly), hemorrhage, peptic ulcers, perforation, small bowel enteropathy, and esophageal ulcerations and maybe appendicitis.
Renal, including decreases in renal blood flow. High doses of NSAIDs in some high risk groups has (rarely) produced acute renal failure.
Hematologic, such as increased blood fibrinolytic activity. Rare cases of hypoprothrombinemia, thrombocytopenia, thrombocyturia, megaloblastic anemia, and pancytopenia. I have no idea what any of those mean, but they don't sound fun at all.
Hypersensitivity, including bronchospasm, rhinitis, conjunctivitis, urticaria, angioedema, and anaphylaxis. Commonly a problem amongst asthmatics.
Dermatologic, including Stevens-Johnson syndrome and a lichenoid eruption.
Hepatic, including hepatotoxicity and cholestatic hepatitis.
Oncologic, maybe pancreatic cancer, which is a particularly horrible cancer.
Metabolic, including dehydration and hyperkalemia.
Cardiovascular, including salicylate-induced variant angina, ventricular ectopy, conduction abnormalities, and hypotension.
Nervous system, including agitation, cerebral edema, coma, confusion, dizziness, headache, cranial hemorrhage, lethargy and seizures.
Musculoskeletal, including rhabdomyolysis.
Respiratory, including hyperpnea, pulmonary edema, and tachypnea.
Endocrine, including hypoglycemia and hyperglycemia.
Ocular, including localized periorbital edema.
Wikipedia does a reasonable job here too, explaining more of the background.
Now that you are totally scared off from aspirin, please try to remember that these are generally either low risk or minor. The benefits of aspirin for occasional use, or where the stroke/heart risks are heavy, is probably warranted. However, these are serious considerations before popping them every day just for their prophylactic effects on Alzheimer's.