Question: what causes drugs to affect people differently?

Fiona Scott answered on 12 Mar 2019:

We’re all different! While the vast majority of our DNA codes for the same enzymes etc. we are all slightly different. This means drugs sometimes work in some patients but don’t for example. Cancers are all different as well. We can generalise as much as we can but increasingly, with greater understanding of our genetic make up we can predict which drugs will work in which patients.

Take breast cancer for example: There are types of breast cancer that have special hormone receptors on the outside of the cell but there is also a type called “triple negative breast cancer”. This means the cells lack those receptors which mean hormone drugs that would normally fit into those receptors (kind of like baseball gloves, ready to catch a molecule) wouldn’t work.

My friend who is currently undergoing treatment for triple negative breast cancer was able to skip trying these hormonal drugs first because a genetic test showed her cancer was this “triple negative” variety so they skipped trying those drugs and moved on to the medication that has currently been shown to be most effective in triple negative breast cancer cells.