Question: why do you become immune to medication?

David Foley answered on 10 Nov 2014:

Good question. There are many reasons and it often depends on the type of illness you have.

Take antibiotics for example. You may have read in the news about antibiotic resistant bacteria or “superbugs” in hospitals. These bacteria have developed ways to quickly destroy the antibiotic as soon as it reaches them. The bacteria that can do this now have an advantage over their neighboring bacteria that cannot – they therefore outgrow them and continue to make you ill. This is the basic principle behind evolution – survival of the fittest.

Cancer is similar. Most drugs to treat cancer do so by killing the cell. Sounds sensible, but doing that creates what we call “selection pressure”. If a single cancer cell can find away to avoid being harmed by the drug, it will survive and outgrow its neighbors. Cancers usually do this in the same way as bacteria, by altering the enzymes inside them to destroy, remove or compensate for the presence of the drug.

Other times you become “tolerant” to a medicine – meaning you require ever higher doses to see an effect. Pain is a good example. Patients in extreme pain are often given morphine. It works well initially, but out body compensates by producing more of the targets (opioid receptors) that morphine works on. So you need more morphine to stop the pain – which causes more receptors to be made – and so on. A dramatic example of this comes from heroine addicts – they can tolerate large doses of the drug that would kill a non-addict because they have become tolerant.

Newer drugs called biologics also become less effective over time. This drugs are different to small molecule (chemical) drugs in that they are large proteins or fragments of antibodies. They usually become less effective over time because our own immune systems begins to attack them as foreign invaders. An example here is Humira – an arthritis drug that is only effective 50% of the time and gradually becomes less effective due to the patients own immune response.

Hope this is all clear!