Numbers Needed To Treat Or To Harm


When it comes to medicine, I would hazard a guess to say that almost every patient I have ever seen has never heard of this concept. As well, I would guess that almost every non-medical friend and family member of mine, does not understand these very important facts. NNTs (Numbers Needed To Treat) and NNHs (Numbers Needed To Harm) are the most important numbers in the world of scientific medicine for clinical relevancy. Concerning your health and medical treatment, they are fabulous guidelines to make wise decisions from.

You know how some breaking news stories will exclaim, "Treatment X decreases the risk of Disease A by 50%!!!"  Not to mention, "40% of the time, Drug Z is effective for curing Condition B!!!"

I mean, doesn't that sound mighty impressive?!?! Hell, if that were really true, I'd be waiting in line for Treatment X and Drug Z.

But alas, what these news stories give you, are RRRs (Relative Risk Reductions), not the all important ARRs (Absolute Risk Reductions).  And like Dr. James McCormack's music video in my previous blog sings out, "Relative numbers they can wait, I want absolute numbers on my plate".

I'll break it down by giving you a simplistic example.

There are 100 people in a study that have Condition B. They are randomly selected and are given Drug Z. Both they, and their nurse, do not know whether they are receiving just a sugar pill (placebo) or Drug Z.

There are also 100 other people in this same study that have Condition B. They are also randomly selected and are given a sugar pill instead of Drug Z. They don't know they are getting the placebo and neither does their nurse.

At the conclusion of the study after all of the number crunching is in, it is determined that 2 people out of 100 from the Drug Z group have experienced a cure in their condition. It is also determined that 1 person out of 100 from the placebo group has also experienced a cure in his condition.

2 cures (Drug Z group) as compared to 1 cure (placebo group), whoa! That's a 50% RRR. Pretty potent drug, huh?  Not quite.

If we could dip into quantum mechanics and be in two different places at the same time, then the 'you' who is taking the drug is twice as lucky to get cured as the 'you' who is taking the placebo.  But since humans can't be in two places at the same time, the relative meaning in Relative Risk Reduction is meaningless.

A more meaningful way of looking at this data, is that the Absolute Risk Reduction (ARR) will tell you that only 2% of the people in the Drug Z group were cured as compared to the 1% in the placebo group. That's a reduction of a big whopping 1%.

Can you imagine THESE headlines?  "1% of the time, Drug Z is effective in curing Condition B."

If Condition B was a terminally fatal disease that had no known cure, even 1% would be impressive. However, for other non-life threatening conditions, 1% is not impressive. The NNT in this case would be 1. What this means, is that 100 people would have to take Drug Z in order for only 1 person to benefit from it.  In other words, of 99 other people taking Drug Z, how lucky do you think that you would be the one being cured?

That's the NNT part of the story.

The other part of the story, is that in this same study, 5 out of the 100 people taking Drug Z experienced severe side effects. In the placebo group, only 2 out of the 100 people experienced severe side effects.

That means that 3% of people taking Drug Z experienced severe side effects. For every 33 people taking Drug Z, 1 will have a severe side effect and this is the Number Needed To Harm (NNH).  In other words, of 32 other people taking Drug Z, how unlucky do you think that you would be the one person experiencing severe side effects?

As mentioned, depending on the condition, taking a shot at having a chance of 1% benefit with a 3% risk of severe side effects might be a wise move. For other conditions, the benefit:risk ratio would be abysmal. This is evidence-based medicine.

I've been wanting to write this blog for a long time now, but what got me off my butt to write it, is a comment I have heard going around this town for over a decade now. I admit, it's second hand information so I did not hear this directly from the specialist himself. However, over the years, I would say that I have heard some version of this comment from over 50 women. It goes like this:

"Every woman should be on the birth control pill or HRT. I would put all of the women in my family on this."

Well, you know me.

I'm a big proponent of ONE SIZE DOES NOT FIT ALL. But the last time I heard a female patient say some version of this comment to me, that was the last straw. I just had to look at the data that made this specialist so sure that every human being with a vagina and boobs needed their daily regular dose of oral contraceptives.

Make no mistake. Oral contraceptives (OC) are a phenomenal therapy for so many reasons, including fertility control and dysfunctional uterine bleeding absent gynaecological pathology, to name a minuscule few.

I presumed the impetus of this specialist's statements was based on the idea that OCs prevent ovarian cancer (an often deadly cancer because it is often diagnosed in its later stages), endometrial cancer and colorectal cancer.

I looked at the data. Not impressive.

At first blush, in this study, one of the data points states that the RRR for ovarian cancer prevention, is that 50% of women will be spared this horrible disease if they use OCs for 15 years. But we already know about RRRs, don't we?

However, when you get down to the nitty gritty and look at the data, it's not as jaw dropping as a 50% RRR. The NNTs go like this:  500 women need to take OCs for 10 years in order that there will be 1 woman who will benefit from the prevention of an ovarian cancer death.  And 250 women need to take OCs for 10 years in order that 1 woman will benefit from the prevention of ovarian cancer.

Now, if a woman is taking OCs for a darn good reason anyway, this information is just possible bonus protection.  If she has a family history of ovarian cancer, it may be an excellent consideration to start taking OCs.

But for every woman to take OCs carte blanche for the prevention of ovarian and endometrial cancers . . . well, the science is not super robust for that kind of ONE SIZE FITS ALL mentality.

Not to mention, there are rare risks to taking OCs like VT (with rare fatalities) and a negligible increased risk in breast cancer.

Every woman is unique. Every woman is different. And every woman deserves medicine that is not shaped in a cookie cutter type of fashion.

Along with OCs, (that I do not prescribe carte blanche), I also do not prescribe multi vitamins carte blanche, because I have looked at the data. The most hilarious commentary based on the science (NNTs) of multi vitamin consumption was co-authored by Dr. McCormack. Take a read. It left me rolling on the floor:

Sure Fire Tricks To Get The Most Out Of Your Vitamin Supplements

In order to determine if a drug or treatment that is prescribed for you is in your best interest, an excellent (but very incomplete) website to start with is called The NNT.

As my favourite video of all time sings out:

"Clinical evidence is the maxim, NNTs are now my passion".