Chloramphenicol is used to treat bacterial conjunctivitis
The move to sell antibiotic eye drops over the counter has led to a large rise in usage, prompting fears about drug resistance, a study says.
Oxford University found that two years after the change, 3.4m doses of chloramphenicol, a conjunctivitis drug, were being sold annually - a 50% rise.
Researchers said the trend was "concerning" as the problem often cleared up without the need for drugs.
But doctors said the move had improved patient access and freed up GP time.
The Medicines and Healthcare Products Regulatory Agency reclassified the eye drops in 2005, allowing pharmacists to hand them out without the need for a prescription.
While the following two years saw a slight drop in GPs handing them out, 1.5 million started accessing the drops from pharmacists, according to IMS Health, a company which monitors drug supplies.
The researchers said the findings, published in the British Journal of General Practice, had important implications for bacteria-acquiring antibiotic resistance.
They said there was already some signs of resistance in these drugs and warned this may be the "thin end of the wedge" for over-the-counter drugs.
Since the decision on eye drops, treatments for chlamydia and urinary tract infections have been made or look likely to be made available without the need for a prescription.
Dr Peter Rose, the lead researcher and a practising GP, said: "At a time when we are encouraging less reliance on antibiotics, the rise we found is concerning.
"We need to see the NHS and patients take more care with the use of drugs."
But Dr Jim Kennedy, the prescribing spokesman for the Royal College of GPs, said: "I would be more worried if the rise was much higher, perhaps two or three times.
"Patients now have better access to drugs and it has freed up doctors a bit. It is something we should keep monitoring, however."