Which of the following is not a focus of Phase 3 clinical testing?

Phase 3 clinical testing serves a critical role in the drug approval process, primarily focusing on evaluating the drug's effectiveness and safety in a larger population. This phase is designed to provide comprehensive data on how the drug performs in comparison to a control group, which may be a placebo or current standard treatment. The results from these tests are crucial for regulatory approval and for understanding the drug's overall benefit-risk profile.

While examining drug interactions is an important aspect of pharmacology, it is typically not the primary focus of Phase 3 trials. Instead, detailed assessments regarding drug interactions are often explored during earlier phases or in post-marketing studies, as Phase 3 primarily aims to confirm the drug's efficacy and monitor its safety across a broader demographic before it enters the market.

In summary, while drug safety, comparison with a control group, and evaluating drug effectiveness are fundamental elements of Phase 3 clinical testing, examining drug interactions is not at the forefront during this stage, thus making it the correct choice for what is not a primary focus.