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Post hoc ergo propter hoc!

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Rags Methanol tries to understand patients’ sometimes flawed logic

Post hoc ergo propter hoc. For those not au fait with Latin, this translates approximately to “after this, therefore because of this”. It describes a logical train of thought that because one thing occurs after another, it is caused by it. The logic, however, is often flawed. Make sense? Allow me to illustrate.

In a recent phone conversation with a patient as part of the new medicine service, a patient told me that she had stopped taking her ramipril because it had given her a sore elbow. Pharmacology tells us that an ACE inhibitor is more likely to cause a lottery win than localised upper-limb arthritis. Nonetheless, in the patient’s mind: “Last week my elbow didn’t hurt. Then I started taking the new tablet and now my elbow hurts. It must be the tablets.”

Another example was a patient who told me she was allergic to paracetamol. When I quizzed her on the nature of her allergy, she explained: “It gives me a headache.” Still not convinced of her explanation, I enquired further. “Well, a few years ago, I had a headache so I took some paracetamol — but it made the pain worse.” Although implausible to the healthcare profession, the origin of the logic is understandable: “I had a headache, I took some paracetamol, my headache got worse. Clearly paracetamol doesn’t agree with me, I must be allergic.”

The need for a medicine to have a positive effect is important to patients. I speak from experience as a lifelong asthma sufferer. When I inhale salbutamol, it relieves my wheezing and, almost instantly, I feel able to breathe more easily. When I take a cortico-steroid inhaler, nothing happens. It took a four-year pharmacy degree for me to overcome the obvious conclusion that beclometasone did sweet Fanny Adams. What chance do our patients have?

Since its introduction, there has been mixed reaction to the NMS. It is by no means perfect. The arrival of more paperwork to my daily routine was as welcome as a skunk at a lawn party. The funding source, essentially the same as upping your child’s weekly allowance by docking their daily pocket money, seems a little unfair. In addition, given its painful slowness, I often wonder whether the Pharmabase website is, in fact, powered by a mob of Russian meerkats.

Nonetheless, what the NMS is doing is providing pharmacists with a window of opportunity to identify bizarre misconceptions before patients’ irrational logic becomes deep seated. In a recent medicines use review, a patient admitted that he had not taken his simvastatin for a couple of years because he liked to read the morning paper while wetting his whistle with a glass of grapefruit juice. However, he didn’t want to upset his GP so merrily continued to order the tablets regardless before ing them to his wheelie bin. Having felt no different after taking statins for a couple of weeks, he conceived no risk in stopping them and, presumably, felt the wheelie bin’s cholesterol problem was bigger than his own. Had the NMS existed when he was first prescribed the tablets, perhaps this issue might have been picked up sooner.

I believe the NMS concept is a good one. It has been particularly useful for helping patients who struggle with new inhalers and for identifying patients experiencing side effects to certain medicines, especially calcium-channel blockers and ACE inhibitors. (I am less convinced of the benefit of phoning patients just to check they are coping with taking an aspirin every day.) Moreover, patients seem to appreciate that someone is concerned enough for their welfare to ring them at home.

We will never eradicate irrational patient logic completely; every patient is different and two minds can interpret the same message in very different ways. For every one that complains that his medicines are being changed to a cheaper brand, there appears to be another concerned that her GP is using her as a guinea pig to trial some expensive new medicine.

Research tells us that half the medicines we dispense may not be being used as our medical colleagues intended them. In part, the fact that patients develop misconceptions based entirely using “post hoc ergo propter hoc” logic will contribute to this problem. Identifying these misconceptions, and understanding the logic behind which they were created, is half the battle.

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