Drug side effects and other unintended consequences of medical treatment may be killing and hurting more people than we thought

WHAT is the third leading cause of death in the developed world? Given that cancer and heart disease top the list, you might hazard a guess at diabetes, stroke or car accidents. You’d be wrong. The answer is “iatrogenic” deaths – those caused by medical errors, adverse drug reactions or hospital-acquired infections.

For all modern medicine’s ability to alleviate suffering and prevent premature deaths, it also causes plenty of both. Many medical interventions turn out to have unintended and negative consequences that often emerge as a result of research into better treatments. While many are obvious, we are now starting to uncover more insidious effects.

For all medicine’s success at alleviating suffering and death, it also causes plenty of both

Take cancer. Anyone with experience of chemotherapy knows how dangerous its side effects can be: it can weaken a patient’s immune system to the point where they succumb to mundane infections. That is well understood, but recent discoveries about tumour cells’ ability to spread to other tissues suggest another – chemotherapy might sometimes make cancer more aggressive (see “Giant leaps of evolution make cancer cells deadly“).


Or consider the humble painkiller. Millions of people take them to make bouts of flu more bearable. But at a population level they may be turning infected people into more efficient virus spreaders, causing up to 2000 extra flu deaths in the US alone each year (see “Popping pills for flu fever might make things worse“).

More speculatively, evidence is emerging that gut bacteria can exert considerable influence over our mental health (see “Psychobiotics: How gut bacteria mess with your mind“). If the effects seen in animal studies are replicated in humans, we may have to rethink practices ranging from antibiotic use to screening procedures for transplants.

None of this suggests that chemotherapy, painkillers or antibiotics do more harm than good overall. And the research that reveals their effects will help us to develop still more effective treatments. But it does highlight the need to think broadly about iatrogenic deaths.

According to the most widely cited figure, 225,000 people die this way in the US each year – but that dates from 2000. We won’t know if we’re fulfilling the most basic tenet of medicine – to do no harm – until we start counting more carefully.

This article appeared in print under the headline “Two steps forward”