You may have heard about a meta-analysis, just published in the Lancet, showing that daily aspirin use is associated with a reduction of cancer risk of about 20 percent over a 20 year period. This sounds exciting, and it is.
Aspirin -- which deactivates an enzyme associated with inflammation -- offers a plausible means of reducing the inflammation that is part of cancer progression. Effects of aspirin were strongest for cancers of the gastrointestinal tract, where aspirin exerts its most direct effects. This study reaffirms a potential role for aspirin in both cancer prevention and treatment, and suggests new directions for anti-cancer drug development.
But I do not think the study is necessarily cause for you to start taking aspirin. Here are some of the reasons:
First, as noted, this was a meta-analysis -- a study that pools data from other trials. As such, it can only look at readily available data, the study is not generating any. This study looked at cancer reduction, but did not specifically explore adverse effects of aspirin -- related to gastrointestinal bleeding in particular. Other studies have done so, and generated estimates of risk increase considerably larger than the cancer risk decrease shown in the current study. Before signing up for any medical intervention, we need to know not just potential benefit, but potential risk -- and the risk/benefit ratio.
Second, when you hear "20 percent risk reduction" you should always be inclined to ask: 20 percent less than what? The cancers that were impacted by aspirin use are not rare by any means, but they are also not very common. Over roughly 20 years, colorectal cancer -- the variety most affected by aspirin use -- developed in 2.8 percent of the study participants. So a 20 percent reduction in risk means 20 percent less than 2.8 percent. In other words, the use of aspirin daily for years reduces your risk of cancer from somewhere around 3 percent on average to somewhere around 2 percent on average. This conversion from relative risk -- the big number -- to absolute risk -- the more relevant number -- is a necessary step in interpreting most medical reports. And we can personalize this further: if you take aspirin daily for the next 20 years or so, starting today, and don't develop colorectal cancer -- the probability that would be the case without daily aspirin is around 97/98, or 99 percent.
Some details in the new study are associated with larger risk reductions than the above, but the basic interpretation is the same in every case: there is some potential benefit of aspirin use, but it is measured as a reduction in a risk that is already rather modest in most people. Many more people taking aspirin for 20 years to prevent cancer will not experience that benefit, than those who will.
And, finally, there are other ways to reduce cancer risk that are known to have even greater benefits, along with less risk. Physical activity, tobacco avoidance, eating well and controlling weight are collectively associated with as much as a 60 percent reduction in overall cancer risk, and the side effects are ... better overall health! Of course, this list is harder to 'take' than an aspirin (or two), but it is clearly the better medicine!
The bottom line is this: The study is by no means a blanket recommendation for aspirin use to prevent cancer. If, however, you have risk factors for, or a family history of, gastrointestinal cancers, the likely benefits of aspirin for this purpose might well outweigh the risks. You should discuss this with your doctor. Aspirin to reduce cancer risk seems to me a very reasonable option for some people, but the risk/benefit calculation needs to be personalized to you.
For the rest of us (I will not be taking daily aspirin), this is a more general advance -- shining a new light on cancer, what promotes it and what fights it. That kind of insight leads to more insights, and in the end, will help us master cancer entirely.
Regardless of the role of aspirin in your personal efforts to avoid cancer, you should put the power of lifestyle to good use. Sometimes hard to swallow, I know -- but it remains the best medicine we've got.