Drug companies will soon be required to include the list price of medications that cost more than $35 and are sold through Medicare or Medicaid in its television advertisements thanks to a federal rule finalized May 8.

Now industry observers are waiting for the other shoe to drop: namely, a lawsuit to stop the rule in its tracks. The lobbyist group representing brand pharmaceutical companies insinuated it would sue when the Centers for Medicare & Medicaid Services first proposed the...