July 18, 2017, by Editor

Why some drugs don’t work for pain

This blog has been written by Associate Professor Roger Knaggs.  He has been instrumental in the development of PAIN (Pharmacist Analgesia Interest Network) and he is chair of the United Kingdom Clinical Pharmacy Association’s Pain Management Group. Following co-option to the British Pain Society Council for several years, he became the first Pharmacist to become an elected Council member in June 2011.

Codeine is one of the most commonly prescribed pain-relieving drugs for mild to moderate pain. In 2016 there were over 20 million NHS prescriptions written for codeine, including co-codamol (codeine and paracetamol). However, it is not effective for a considerable proportion of people.

The problem is that an estimated 7 to 10 per cent of the population have a gene variation which means they don’t make enough of an enzyme in the liver, called CYP2D6, to obtain pain relief from codeine. Up to six million Britons may be affected in this way, although most are unaware of it. Patients often keep taking codeine and hope that it will work eventually —however all they experience are side-effects, such as constipation and none of the pain-relieving benefits.

But that is not the only issue with this enzyme. There is another group of people who produce too much CYP2D6, so codeine is broken down too quickly in their bodies. This creates a fast ‘hit’ which then rapidly wears off. Blood tests to measure CYP2D6 activity have been available for about 30 years but they have been very expensive. Although the costs have reduced they are not used routinely and are really only used in research studies.

People are often shocked to learn that drugs don’t work for everyone, but all drugs are metabolised differently. If you gave ten people the same drug you’d get a different response in each. However, even if you don’t have a problem with production of the CYP2D6 enzyme, codeine still might not work for you, as the way each individual responds to pain is such a complex process. So if codeine isn’t working for you, tell your doctor so that alternative pain-relieving medicines can be explored.

Dr Roger Knaggs

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