It is pretty self evident that any shortages that are present in existing drug supplies are going to be exacerbated by crashing out of the EU, the customs union and the single market.

Arguing that current shortages under the existing system mean that additional shortages caused by leaving it can be disregarded is analogous to climate change deniers who argue that, if natural processes are a factor in causing climate change, there is no point in reducing human activities that contribute to these changes. Essentially, don't worry about adding fuel to an existing fire.