Five New Territories farms are being investigated on suspicion of using the banned asthma drug clenbuterol on pigs.
Tests on 1,150 pig urine samples were done by the Agriculture and Fisheries Department between August 14 and 27, and seven pigs from five farms were found to have been given the drug.
Clenbuterol is a growth stimulant that reduces fat and leads to leaner meat.
The dosing sparked a scare in the mainland this year when dozens of people were treated after eating pork containing clenbuterol.
It can cause nausea, dizziness, headaches, tremors, cramps and heart palpitations.
Several Hong Kong people also fell ill after eating tainted offal and pig's lung soup.
Mainland produce was blamed.
The Hong Kong pigs have been traced by a new branding system introduced by the Agriculture and Fisheries Department which matches the pig to its farm.
Since the testing began earlier in the month, 106 samples have tested positive and 22 farms have been raided. A department spokesman said four farm suppliers had also been raided.
When enough evidence had been gathered, the Department of Health would prosecute farms or suppliers believed to be using the drug.