An investigation has been launched into suspicions that medicine from a public hospital ended up for sale on an e-commerce platform, authorities said on Wednesday.

On Tuesday, HKTVMall said it had removed a merchant from its online marketplace after a buyer complained that packaging for an order came with a label indicating the product had originally been given to a patient of Hong Kong Buddhist Hospital.

The product is a laxative which is not classed as a poison and does not require a prescription or for sales to be supervised by a pharmacist, the government said in a statement.

Nevertheless, the administration expressed "grave concern over the suspected resale of public hospital patients' medicines" and said the matter will be dealt with seriously.

"The DH (Department of Health) is liaising with the Hospital Authority to seriously investigate into the alleged case and shall take prompt enforcement action as appropriate," a spokesperson said.

"For this specific case, the DH will investigate along the line of illegal sale of unregistered pharmaceutical products."

On an RTHK programme, So Yiu-wah, president of the Society of Hospital Pharmacists, suggested that "over-prescribing" of medicines by public hospitals could have played a role in the alleged incident.

Patient rights advocate Tim Pang, from the Society for Community Organisation, said doctors at public hospitals tend to dish out too much medication because of the length of time patients have to wait for their next consultation.

Pang said more community pharmacies would solve this problem.

"Because if there are enough community pharmacies for patients to refill the drugs, then the public hospital doctors can prevent from prescribing a lot of drugs to the patients during that waiting period," he said.