Forgot where I first read about this but the Lancet reported that taking aspirin daily not only reduces heart disease risk but also reduces the risks of a plethora of cancers. The abstract can be read here.
What kinds of questions come to my mind when I read something like this?
First and foremost, I note that this is -- at this stage -- a purely statistical finding, not (yet) backed up by a biological mechanism that can explain why aspirin reduces cancer risk. One can be much more comfortable with the result if the biology were there to support the statistics. (This is a theme in Chapter 2 of Numbers Rule Your World.)
Secondly, one always should be wary when one drug is said to cure a dozen ailments. In such situations, one must ponder whether there is a hidden cause lurking behind all of this. For example, it could be that people who take aspirin daily are self-selected, and are more likely to take better care of their health.
The feel-good factor of this finding is that the words "randomized trials" in the title of the paper. It gives the impression that the aspirin test group and the placebo control group were randomly selected and thus self-selection is not an issue.
At this point, we need to delve into what kind of analysis was done. It is a "meta-analysis", meaning that they aggregated the results of "eight eligible trials", each of which was a randomized trial, and treated the pooled data as if it were a large randomized trial. Presumably each trial had different enrollment rules, and so it is not assured that the pooled groups are unbiased. Would like to see if they looked into this.
The key passage is this:
On analysis of individual patient data, which were available from seven trials (23 535 patients, 657 cancer deaths), benefit was apparent only after 5 years' follow-up (all cancers, hazard ratio [HR] 0·66, 0·50—0·87; gastrointestinal cancers, 0·46, 0·27—0·77; both p=0·003). The 20-year risk of cancer death (1634 deaths in 12 659 patients in three trials) remained lower in the aspirin groups than in the control groups (all solid cancers, HR 0·80, 0·72—0·88, p<0·0001; gastrointestinal cancers, 0·65, 0·54—0·78, p<0·0001), and benefit increased (interaction p=0·01) with scheduled duration of trial treatment (≥7·5 years: all solid cancers, 0·69, 0·54—0·88, p=0·003; gastrointestinal cancers, 0·41, 0·26—0·66, p=0·0001).
I'm surprised that they were able to keep the placebo/control groups in all of these 7 or 8 trials clean for 20 years. Is it really true that none of the patients in those groups took aspirin all those years? They would have a much more restricted result otherwise.
In addition, reading this passage (and other parts of the abstract), it is clear that the researchers conducted an exploratory data analysis. They told us it worked for this type of cancer but not another type, after this many years of delay but not fewer, etc. For this type of work, the conventional p< 0.05 criterion is inappropriate; the standard of significance at the level of individual tests must be much, much tighter. This is especially true of "meta-analysis": pooling together test populations will always reduce the margin of error but I'd like to see corrections for multiple comparisons (or, as Andrew Gelman likes to argue, a sophisticated modeling regime that shrinks the effects).
If I were the authority, I would not recommend any treatment strategies based on this paper but I'd order up confirmatory tests to validate the hypothesis directly, and also order up research to uncover the biological mechanism that links aspirin to curing these cancers.