Discount shops Poundland and 99p Stores have come under fire for selling three packs of painkillers for £1.
The Medicines and Healthcare Products Regulatory Agency said most major UK retailers had signed up to its voluntary code to sell no more than two packs of 16 painkillers in a single transaction, but Poundland and 99p Stores "refused to co-operate".
Restrictions on pack sizes for analgesics were introduced by the government in 1998 to reduce the potential for abuse or excessive use of medicines and accidental poisoning in children.
"The MHRA strongly discourages the sale of multiple packs as this undermines the intention of the pack size restrictions, which are designed to save lives," said Jeremy Mean, MHRA policy group manager.
"We are deeply disappointed that Poundland and 99p Stores have consistently failed to agree responsible sales policies. The government has made clear it will consider changing the law if retailers do not act responsibly."
A spokesman for 99p Stores said it was reviewing its policy in the wake of the MHRA's criticism.
Poundland, however, remained defiant, insisting it was acting responsibly. "Our policy conforms with current UK legislation," said CEO Jim McCarthy.