The owner and two employees of a Mongkok pharmacy were arrested for selling controlled drugs to a rookie policeman disguised as a student in an undercover operation last night. The raid came after police received information that the Dundas Street shop would sell controlled drugs - cough medicine and tranquillisers - to students in school uniform able to quote a secret code number, Senior Inspector Wong Cho-sing of Mongkok police station said. The pharmacy, which had been open for more than three years, had started selling the controlled drugs after it went into debt recently, he said. Officers believed hundreds of students had bought drugs from the shop in the past month. Surveillance showed students, mostly from Mongkok schools, bought the drugs around 4pm each day. An undercover agent wearing a school uniform was sent to the pharmacy at 6pm. Eight officers led by Mr Wong and a Department of Health official waited outside. The arrests were made when the undercover officer managed to buy the drugs without a doctor's prescription. Officers seized more than 400 tranquilliser tablets and several bottles of cough medicine. Controlled drugs must be kept under lock and key and be dispensed only by a pharmacist. Last night, the 65-year-old owner and his two employees, aged 23 and 55, were being held for questioning. Police said the three were likely to be released on bail pending investigations.