A 52-year-old man with type 2 diabetes mellitus is enrolled in a randomized controlled trial comparing a new antidiabetic agent (Drug X) versus metformin for glycemic control. He is randomly assigned to the Drug X arm. At baseline, his HbA1c is 8.2%, fasting blood glucose 156 mg/dL, and BMI 28.5 kg/m². After 12 weeks of treatment, his HbA1c drops to 7.1%, and he reports excellent adherence. However, the trial coordinator notes that 18% of participants in the metformin arm discontinued the study due to gastrointestinal side effects, while only 2% in the Drug X arm withdrew. Which of the following best describes the primary threat to the validity of this trial's findings?