Advice concerning the sale of paracetamol is being contravened, according to a snapshot study from one London hospital.
Paracetamol packets are restricted in size
The UK drugs watchdog recommends only one pack of the common painkiller should be sold at any time.
But a study in the Postgraduate Medical Journal has found some outlets were willing to sell more than this amount.
And some patients brought into A&E for paracetamol poisoning said they had purchased more than one pack at a time.
Legislation introduced in 1998 restricted the size of paracetamol packs in an effort to curb deaths from overdose.
It declared pharmacies could only sell packets of up to 32 tablets, while other outlets such as supermarkets or corner shops were limited to packs of 16 tablets.
The Medicines and Healthcare Products Regulatory Agency (MHRA) also recommended only one packet of paracetamol should be sold at a time.
But a study from scientists based at Guy's and St Thomas' NHS Foundation revealed outlets were not always adhering to this advice.
In 2004, researchers visited 24 pharmacies and shops and attempted to buy large volumes of paracetamol tablets.
They found 70% of the outlets allowed them to buy more than one packet of paracetamol in one sale, while 13 of the shops or pharmacies let them purchase 48 tablets or more.
The team also interviewed 107 people brought into the St Guy's and St Thomas' emergency department for paracetamol poisoning.
Half of the 77 patients who had swallowed more than 16 pills said they had deliberately set out to buy paracetamol for an overdose.
And of these, 16 people (46%) said they had purchased more than one packet.
Researcher Dr Paul Dargan, of the hospital's poisons unit, said: "A significant number of patients were able to buy significant does of paracetamol from a single outlet.
"And when we tested this for ourselves by going to outlets to see how easy it was to buy two, three, or four packets of paracetamol, we found again that in a significant number of outlets it would be easy to do."
"We are clinicians and toxicologists and we are concerned the legislation hasn't done its job, which was to reduce the availability of paracetamol and to reduce the likelihood of patients taking significant overdoses."
The authors emphasised this was a small study in one area.
Another study carried out in 2004 found fatal overdoses had been reduced by 25% after the legislation was introduced.
Paul Farmer, chief executive of mental health charity Mind, said: "If this pattern is repeated throughout the country then there is cause for concern here.
"The introduction of new regulations on paracetamol purchasing has been successful in lowering the number of people who have died due to an overdose.
"It would be very worrying if these regulations were being forgotten or ignored, and it's important that retailers, who often have high staff turnover rates, are regularly reminded of their responsibilities."
A spokesman from the MHRA said: "The MHRA has worked with the Over The Counter (OTC) medicines sector and major retailers to promote voluntary restraints on multiple sales, such as till bars on purchase of more than two packs of analgesics and avoidance of multiple sales promotions.
"The MHRA continues to review the need for additional measures to protect public health."