Amlo chief Seehanat Prayoonrat said investigators are also probing the suspected drug rings which receive the drugs from Thailand.
Preliminary results show gangs are paying hospital staff to order the drugs, which are then diverted from hospital pharmacies and sent by mail to northern provinces.
There, drug members intercept them and take them across the border.
Tharit Pengdit, chief of the Department of Special Investigation (DSI) said methamphetamine and crystal methamphetamine are produced out of chemicals synthesised from the pseudoephedrine-based pills.
The drugs are then smuggled into Thailand once again, he said.
The pseudoephedrine-based drugs which gangs rely on for making illegal drugs are both legally imported by hospitals and illegally smuggled into the country, he said.
One cold relief pill can be turned into three methamphetamine pills.
The DSI is also investigating unusual transactions by some doctors and pharmaceutical companies that had opened several clinics and ordered a large amount of cold relief pills, a DSI source said.
The DSI was establishing a link between these doctors, drug firms and certain drug rings, the source said.
Meanwhile, police say five staff members of Kamalasai Hospital in Kalasin are thought to be linked to the disappearance of 356,535 tablets of pseudoephedrine-based drugs from the hospital.
The suspects, including a pharmacist, were found to have been involved in falsifying at least 21 out of 37 reports of pseudoephedrine-based drugs prescribed to patients, Pol Col Wichian Pinduang, deputy provincial police chief, said.
Another 20 drug use report forms had gone missing, he said.
The Public Health Ministry expects to conclude its investigations into alleged smuggling of pseudoephedrine-based drugs at four of its hospitals this week, Dr Paijit Warachit, secretary permanent for public health said.
A pharmacist suspected of being involved in alleged cold-drug smuggling at Udon Thani Hospital was likely to be dismissed this week, Dr Paijit said.
Cold pills are also thought to have gone missing from Kamalasai Hospital in Kalasin, Thong Saen Khan Hospital in Uttaradit, and Doi Lo Hospital in Chiang Mai.