The truck carrying the consignment of syrup bottles and some other items were intercepted by a BSF check squad, a senior official of the paramilitary force said.
The consignment, valued at Rs 39.76 lakh, was to be smuggled into Bangladesh last night, the official said.
Mohammed Naimuddin Sikdar (29), driver of the truck and Jahir Hussain (26), both residents of Garia Satla village in Assam's Barpeta district, were held, the official said.
Phensedyl contains Codeine, an opium derivative, and this medically prescribed anti-cough tonic is widely abused in Bangladesh, India and other parts as a narcotic stimulant.