" It is no simple matter to rework sources and delivery paths."
Quite possibly true. But as said elswhere, it is the function of government to plan for all eventualities. The time frame for seeking alternative sources and routings has existed since the referendum result was known. There should be no excuse for not achieving a painless transition unless the plans contain undetected errors.
The recent scare stories about possible medicine shortages is really more a case of EU pricing structure and a a new regulatory system within the EU taking place in 2018. Brexit is merely the icing on the cake and heavily rationed icing at that!