"I feel better, so it's done its job"

This is the belief that stops antidepressants at three months, preventer inhalers in the summer, and the tablets for an ulcer as soon as the pain has gone. It's the most natural belief in the book, and for a lot of medicines it's exactly backwards.

The holding ones work by holding. A preventer inhaler keeps the airways from being inflamed, and while it's doing that you have no symptoms, and the absence of symptoms is the medicine at work. An antidepressant that has started to lift your mood is doing so in the present tense, and the NHS says you'll usually carry on taking it for at least six months after you feel better, because feeling better at month three is where the treatment is, not where it ends. Levothyroxine for an underactive thyroid replaces a hormone your body isn't making, and the NHS page says plainly that you'll usually take it for the rest of your life to stop the symptoms coming back. For all of these, "I feel better" and "it's working right now" are the same sentence.

The trouble is that the feeling doesn't come with a label saying which. When you had a headache and took a painkiller and it went, the medicine had finished. That's the model most of us carry, because it's the medicine most of us take most often. The holding ones don't behave like that, and nothing about the feeling tells you so. A study of 405 adults with asthma found that adherence to preventer inhalers ran at about half, and that each 25 percent increase in the time spent without the inhaler doubled the rate of hospital admission for asthma. Those people weren't ignoring symptoms. They'd stopped because they didn't have any.

So the question to ask isn't "am I better" but "is this one of the ones that finishes, or one of the ones that holds". The leaflet usually says, the pharmacist always knows, and chapter 20 is about asking. Feeling better is good news. It just isn't an instruction.