Some of you may have noticed that I have, more than once, written about just how ineffective cholesterol-reducing drugs (including statins) are for preventing heart attacks and stroke and saving lives. Taken over several years, the data show that the vast majority of people who take them will simply not benefit from them (and have a significant risk of side effects too). You can read a couple of recent posts regarding these facts here and here.
Yesterday, I was reading something about blood pressure – another ‘marker’ (along with cholesterol) we’re kinda-obsessed with in medicine. I’ve been aware for some time that salt reduction (vigorously promoted by most Governments and relevant health agencies) doesn’t appear to have any miraculous beneficial effects, but what about drugs for blood pressure so-called antihypertensives? Do these medications deliver on their promise?
I decided to have a look at the research. First thing, I found unearthing relevant data surprisingly difficult. Certainly, there is much less here than we have regarding cholesterol-reducing strategies, and statins in particular. But I did manage to find one quite-recent meta-analysis of 147 trials that has a wealth of data we can sink our teeth into [1].
Overall, the evidence shows that a reduction of systolic blood pressure (the higher of the two blood pressure values) of 10 points (mm of mercury) reduces the risk of heart attacks by 22 per cent and strokes by 41 per cent. Pretty impressive numbers on the face of it.
However, the relevance of these numbers is only clear once we put them in the context of someone’s overall risk. Because if risk is relatively low, then even a significant reduction in relative risk won’t amount to much. The authors of the meta-analysis break the data from the studies into lots of subgroups according to age, starting blood pressure and number of medications. Let’s take a typical scenario and crunch some numbers…
Let’s take someone in their mid-60s who is diagnosed with raised blood pressure. Let’s say it takes two medications to bring their blood pressure down to an acceptable level. The data in the meta-analysis tells us that this would reduce the risk of heart attack by about 40 per cent. Stroke risk reduction would be around 50 per cent.
Let’s look at heart attack risk reduction first. The authors tell us that the 10-year risk for heart attacks in England and Wales is estimated to be about 10 per cent for men and 5 per cent for women.
So, for men, a 40 per cent reduction in over 10 years takes their risk from 10 per cent to 6 per cent. In other words, over 10 years their risk has fallen in absolute terms by 4 per cent. Each year, the risk has fallen by 0.4 per cent in real terms.
From this data, we can calculate the ‘number needed to treat’. So, how many people do we need to treat with two medication for a year to prevent one heart attack in this scenario? To get this figure we divide 0.4 into 100. The NNT is 250. The same calculations for women reveal and NNT of 500.
Now let’s look at stroke. According to the British Heart Foundation, strokes are more common than heart attacks (about 25 per cent more common), and remember the relative risk reduction would be expected to be higher (about 50 per cent rather than 40 per cent). However, when we factor this in we still get astronomical NNTs (about 160 and 320 for men and women respectively, according to my calculations).
Even if we extend these NNTs over time, they still are not impressive. Taking the best numbers here (stroke prevention in men), an NNT of 160 over one year equates to an NNT of 32 over 5 years. So, of 32 men treated, one will avoid a stroke, but the other 31 will not. Which means 97 per cent of men taking two medications over 5 years will not benefit in terms of stroke prevention.
Now, of course, these slim benefits need to be weighed against the potential adverse effects of antihypertensives (of which there are many, unfortunately).
I not a naturally negative person, I think, but it seems to me that the data on antihypertensives is similar to that on statins. In real terms, the benefits appear to be much smaller than we’re led to believe, and the medications carry real risks too.
Despite all of this, the authors of the meta-analysis are uber-enthusiastic about antihypertenives and urge us to medicate all people over a certain age irrespective of their blood pressure. They concede that some would say this is ‘medicalising’ a population, but personally have no such concerns. In fact, they suggest that it no more ‘medicalising’ people than, say, vaccinating them or having them take the contraceptive pill.
I suppose it should be borne in mind that the authors of the meta-analysis are Professors Malcolm Law and Nicholas Wald, who own patents for a multi-medication pill (the ‘polypill’) that includes (perhaps predictably) antihypertensives.
References:
1. Law MR, et al. Use of blood pressure lowering drugs in the prevention of cardiovascular disease: meta-analysis of 147 randomised trials in the context of expectations from prospective epidemiological studies. BMJ 2009;338:b1665
