The Government Pharmaceutical Organisation (GPO) has admitted mistakenly putting heart failure medicine into packaging for hypertension drugs.
The error occurred in July, it said.
Newly appointed managing director Suwach Siasiriwattana said the drugs were then distributed to seven state-owned hospitals in Kanchanaburi, Si Sa Ket, Chaiyaphum, Suphan Buri and Lop Buri provinces.
Dr Suwach was responding on Thursday to reports circulated on social media about the mix-up.
He said isosorbide dinitrate, used to treat congestive heart failure, was packed into packaging for amlodopine which is used to treat hypertension.
As a result five patients were prescribed the wrong drugs, he said. There were no reports of any adverse impacts on the patients.
The GPO has already recalled all the drugs that were wrongly packed and is investigating how the mix-up occurred, he said.