Take a medicine and start feeling a bit off, and it’s easy to assume you’re imagining it. Sometimes you’re not.

It could be a known side-effect – the sort of thing listed on the leaflet in the packaging. Or it could be something known as an adverse drug reaction, or ADR.

The terms get used interchangeably, but they’re not quite the same thing. An ADR is an effect that’s new, unexpected, or more severe than anyone anticipated – and it can strike when you start a medicine, when your dose changes, or even after you’ve stopped taking it.

ADRs can happen for many reasons. We all process medicines differently. Your age, your genes and your underlying health can all shape how your body reacts.

Sometimes the dose is simply too strong for you. Or it could be caused by a clash between two medicines, or a medicine and food, or a medicine and a supplement.

And very occasionally, the culprit is the medicine itself: a defective or fake product containing the wrong ingredients, or none at all.

Read more: One in ten medicines worldwide could be fake or substandard – here’s how to protect yourself

Healthcare professionals sort ADRs into several categories, but two matter most.

Type A reactions are the most common. They happen when a medicine’s........

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