It is an inconvience to adults and achieves absolutely nothing in terms of public safety. How many overdoses have been prevented by this imposition on the freedom of the entire population? – zero I bet. It is entirely pointless. What should happen is that the chemist should just apply common sense and perhaps ask customers if they are concerned.
Remove rule that prevents customers from buying more than 2 items containing paracetomal (or similar) at a time. This is a typical nanny state rule, as all you have to do is walk out and back in to the shop or go to another counter to get more.