Gene suppressant

A gene suppressant was a medicine, a bottle of which was kept in the Bank of Karabraxos. It was able to act as an antidote to Saibra's condition of changing form by replicating any living cells upon contact, so she was promised the gene suppressant as a reward for participating in the Twelfth Doctor's plan to rob the Bank of Karabraxos. It was given to Saibra by the the Doctor, who discovered it with Clara Oswald in vault “ORG 339". By the time Saibra was dropped off at home, she had taken it as demonstrated when she was able to hug the Doctor without transforming into him. (TV: Time Heist)