Dubai Customs carried out a major drugs operation, seizing about 800,000 Captagon tablets at the city's busiest port.
Officers with dogs found the Dh3 million stash on a vessel at Jebel Ali.
The pills were hidden inside a fuel container.
Shuaib Al Suwaidi, director of Dubai Customs' intelligence department, said the agency's Risk Engine alert system flagged the vessel as being high risk.
It was scanned from the outside and found to have "abnormal density".
Officers would not identify the ship's home port other than to say it was in an Arab state.
But Syrian drug gangs have been implicated in the manufacture and smuggling of Captagon to the Gulf.
Captagon, initially produced in the 1960s as a treatment for attention deficit hyperactivity disorder, was banned in the 1980s because it can be highly addictive.
Commercial manufacturing of the drug ceased but illegal manufacturing continues. Gangs combine several addictive stimulants that compound the destructive effects of Captagon's amphetamine and theophylline.
