A complete response letter is one of the two usual outcomes of an FDA review, the other being approval. It sets out the reasons the application cannot be approved as submitted, and often what the company would need to do to address them.
The reasons vary. Some are clinical: the evidence of effectiveness or safety is not sufficient, or an additional study is needed. Many are not about the drug’s efficacy at all: problems found at a manufacturing facility inspection, questions about manufacturing and controls, or unresolved issues with the proposed label.
After receiving a letter, a company can resubmit the application with the deficiencies addressed, withdraw it, or request a meeting with the FDA to discuss next steps. Resubmissions are classified by how much new information they contain. The FDA aims to review a Class 1 resubmission within two months and a Class 2 resubmission within six months.
For a competitor, a complete response letter changes the catalyst calendar: a launch that was expected may be delayed by months or years. Companies usually disclose that they have received one, and the reasons they give are worth reading closely.