Shares in Endo Health Solutions took a downturn in after-hours trading last night after it emerged that US regulators have again refused to approve its testosterone injection.

The firm's subsidiary Endo Pharmaceuticals has received a third Complete Response Letter (CRL) from the US Food and Drug Administration, asking for tighter safety measures before it can approve the application to market its long-acting testosterone injection Aveed for the treatment of hypogonadism.

On the plus side, the additional information sought by regulators in this CRL will not involve any additional clinical trials, the firm said.

The FDA is concerned about risks and complications from post-injection reactions, and as such has updated its requirements for a Risk Evaluation and Mitigation Strategy to include a Medication Guide as well as other elements to mitigate these risks.

"We are encouraged by the feedback received from the FDA in the complete response and appreciate the clarity around the path forward for AVEED," said Ivan Gergel, executive vice president of research and development and chief scientific of Endo Health Solutions. 

The company said it plans to submit a complete response by the end of the third quarter of this year.