What's this "pharmacist gag rule" that Trump is talking about?

It won't let me post the link here, but if you Google a TIME article "President Trump's Plan to Lower Drug Prices Spares Pharma Industry" here's an excerpt: In Rose Garden remarks at the White House Friday, Trump called his plan the “most sweeping action in history to lower the price of prescription drugs for the American people.” ... The administration will pursue a raft of old and new measures intended to improve competition and transparency in the notoriously complex drug pricing...Click to expand... What's this "pharmacist gag rule" that Trump is talking about?
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Source Type: forums