Cave: Ignoring the NNT

How would you like to double your chances of winning the lottery? Just buy two tickets!

Statistically, this is true, but is that a reason to spend more money on something that most likely offers no return on investment?

Yet, in medical research, study after study shows impressive improvement in relative risk for this, that and the other intervention but a small or even negligible effect on absolute risk.

For example, I just read a study in the New England Journal of Medicine comparing giving a new osteoporosis drug to women with osteoporosis and a prior history of an osteoporotic fracture for one year, followed by an older drug for one year to just giving the older drug for two years. The two drug regimen lowered an osteoporotic woman’s risk of hip fracture by 38%.

The number of hip fractures in the combination treatment group was 41 out of 2046 patients, and in the single drug group it was 66 out of 2047 patients.

In absolute numbers, treating 2046 patients reduced the hip fracture risk by 25 cases. The number of women one would need to treat to avoid one hip fracture, the “NNT”, is 2046 divided by 25, or 81.

That NNT isn’t terribly impressive, especially in light of the fact that 12 more patients in the new drug group had a cardiovascular event in the first year than in the old drug group.

The editorial accompanying this article does say “In sum, ARCH revealed that romosozumab has great potential as a short-term anabolic treatment for osteoporosis. However, until the cardiovascular and endocrine effects of this antibody are clarified, romosozumab will remain more a part of our expectations than our armamentarium.” But if the drug company starts promoting the relative risk reduction of this treatment, doctors could be misled and patients could come to harm.

Here are some more examples of he Number Needed to Treat for some common health interventions, published in a post I wrote 7 years ago:

1) Shingles vaccine doses given in order to avoid one case of shingles: 59.

2) Ear infections treated with Amoxicillin to avoid one ruptured eardrum: 20.

3) Cortisone shots to relieve one sore shoulder: 3.

4) Aspirin prescriptions to prevent one heart attack: 200.

5) Prostate cancers treated in order to prevent one death: 18-48 (most men with prostate cancer don’t die from their disease).

6) Adenomatous colon polyps removed to prevent one colon cancer: 50 (only 2% of “precancerous polyps” actually turn into cancer).

May I never forget to consider the NNT…

3 Responses to “Cave: Ignoring the NNT”


  1. 1 Mary Symmes October 23, 2017 at 7:46 pm

    I think this accounts for a lot of the over medication in this country. We also are constantly seeing headlines like “New drug makes you beautiful, successful, and wealthy” only to read further and find that that is not actually true. There is such intense anxiety about death – we seem to view it as a preventable illness.

  2. 2 Paul November 6, 2017 at 11:18 pm

    Buying 2 lottery tickets does not double your odds of winning. Statistics are a tricky thing.


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