Final answer:
Toxicities may start to appear in phase 2 trials that were not observed in phase 1 studies due to the larger patient population and a higher likelihood of rare or unexpected toxicities in the target patient population.
Step-by-step explanation:
In phase 2 trials, toxicities may start to appear that were not observed in phase 1 studies. This can happen because phase 1 trials are typically conducted on a small number of healthy volunteers, while phase 2 trials involve a larger number of patients with the target disease.
The larger patient population in phase 2 trials increases the chances of observing rare or unexpected toxicities that may not have been apparent in the smaller, healthier population of phase 1 trials.