Via Quora user Mike Benson:
In 2002, Mucinex filed for FDA approval of its "OTC long-acting guaifenesin product."
"Under the Durham Humphrey Act of 1951, the FDA established that no drug may simultaneously be sold as a non-prescription product and as a prescription product at the same dose for the same indication. Any products that violate this rule are subject to FDA regulatory action and removal from the market."
On Oct 11, 2002, the FDA issued warning letters to 66 manufacturers, distributors, marketers, and retailers of single-ingredient guaifenesin extended-release products that stated that such prescription products require FDA approval, and without FDA approval, they could no longer be marketed legally.
This removal resulted in Mucinex SE being the only long-acting, single-ingredient guaifenesin product available in the United States, according to an SEC 10-Q filing form.