There are concerns over the checks on over-the-counter drugs
Raising the number of drugs that can be bought at pharmacies may affect safety and cut effectiveness, specialists say.
Writing in the British Medical Journal, two doctors say drugs such as statins could be less effective because over-the-counter doses are lower.
They warn some powerful painkillers available without prescription have potentially harmful side-effects.
But the National Pharmacy Association said its members treated newly classified medicines with caution.
The government is keen to see some widely used drugs more easily available, to improve the health of people with long-term conditions and potentially cut the bill to the NHS.
However, West Midlands Centre for Adverse Drug Reactions director Robin Ferner and Keith Beard, from the Victoria Infirmary Glasgow, believe that the benefits need to be closely measured against the risks.
They highlighted two recent deaths in which codeine-based painkillers were implicated at inquests.
They said it was far more difficult for the potential side-effects to be explained to patients in their local pharmacy.
"Certainly, pharmacists can provide clinical advice to minimise the risk of misuse of pharmacy only drugs, but supervision by a busy community pharmacist in the UK may be perfunctory," they wrote.
Patients who misdiagnose their symptoms then try to treat themselves using over-the-counter drugs could delay a vital trip to their GP surgery, they added.
The medicines safety watchdog - the Medicines and Healthcare Products Regulatory Authority (MHRA) relies on reports of side-effects, primarily from doctors, to alert them to any new concerns.
It recently extended its reporting scheme to make it easier for patients to contact them.
However, Dr Ferner and Dr Beard are not convinced that this is enough to ensure their safety.
"Given the concerns, it would be wise to avoid any wholesale rush to reclassify medicines," they said.
"Whole communities might lose out in the long run if indiscriminate overuse of widely available medicines were to lead to large numbers of avoidable but irreversible adverse effects."
A National Pharmacy Association spokesman, which represents community pharmacists, said that its members tended to err on the side of caution when handling newly reclassified medicines.
"When a medicine is available over the counter for the first time, it is generally only dispensed by the pharmacist themselves.
"In fact, when people were surveyed about sources of good quality health advise, as many said they would go to a pharmacist as said they would visit their GP."
A spokesman for the MHRA said: "Patient safety remains the prime consideration in any decision to make a medicine available over the counter.
"We continuously monitor the safety of all medicines in the UK including concerns about misuse, and where necessary, take suitable action to safeguard public health."
The Royal Pharmaceutical Society, which also represents pharmacists, said that pharmacists were well-placed to keep members of the public safe.
A spokesman said: "We support increasing the public?s access to over the counter medicines when supplied under the supervision of a pharmacist. Pharmacists are experts in medicines and are well placed in the heart of communities.
"Patients benefit from the increased choice and convenience these products offer. Pharmacists and their staff are trained to help patients make the right choice."