The Food and Drug Administration has suspended experiments on the effects of nicotine in squirrel monkeys, research aimed at better understanding one of the most pernicious of addictions.

Two weeks ago, British primatologist Jane Goodall wrote to FDA Commissioner Scott Gottlieb, urging an end to what she called “cruel and unnecessary” and “shameful” research.

On Monday, he responded, saying that he had put a hold on the study this month “after learning of concerns related to the study you referenced.” He also said he has sent a medical team of primate experts to the FDA facility – the National Center for Toxicological Research in Arkansas – “to evaluate the safety and well-being of the monkeys and to understand whether there are additional precautions needed.”

The research involved training adolescent and adult squirrel monkeys to press a lever to give themselves infusions of nicotine. Four monkeys in the studies, which began in 2014, have died, according to people close to the situation. The deaths are still being investigated, but nicotine overdose isn’t seen as the likely cause.

Gottlieb also told Goodall that he has appointed an FDA team, including senior career officials and guided by primate veterinarians, to assess the “science and integrity” of the animal research process for the study and whether the research should be resumed. If the study is terminated, he said, the monkeys will be sent to an alternative location that can provide long-term care.