The drug was a cure for a plague that the Ornarans believed would kill them if they did not have the drug. A neighboring planet, Brekka, provided the drug in exchange for food and other necessities. In reality the plague had been wiped out many years before, and the people who died after they stopped taking the drug were in fact suffering from withdrawal symptoms, the medicine being highly addictive. The Brekkians used the drug to control the Ornarans so that they would continue to provide for Brekka.
Romas along with another Ornaran, T'Jon had been beamed aboard the USS Enterprise-D just before their freighter the Sanction exploded. He became involved in a dispute with two Brekkians, Sobi and Langor, who were also on the Enterprise-D, when they refused to give them the drugs, because the goods the Ornarans were transporting were destroyed. (TNG: "Symbiosis")
William T. Riker's memories of the incident, including Romas, were among those "replayed" during an experimental procedure to save his life from an invasive parasite the following year. (TNG: "Shades of Gray")