Because this law is yet another petty and unnecessary restriction of liberty.
Legislation (1998) limits the sale of painkillers to packs of 16 in all shops except pharmacies which may sell packs of 32 to minimise the risk of overdosing. People are not allowed by law to buy more than 32 tablets at one time and some shops display a notice to this effect.
This is a petty and unnecessary law because anybody wishing to buy more than 32 painkillers can simply buy them in more than one shop, or can scan several packs at self-service checkouts in supermarkets. Meanwhile pharmacists are enjoying a monopoly over the sale of the larger packs of painkillers for no good reason.
The Royal Pharmaceutical Society wiil argue that fewer people have overdosed on painkillers since this law was introduced but, even if true,there is no proof that this is due to limiting the amount of tablets people can buy at one time.
I believe this law is less a matter of public safety and more about ensuring the pharmacists' monopoly over the sale of the more lucrative, larger packs of painkillers and should be scrapped accordingly.