Newt is delighted to have his brother visit more often. He knows Theseus’ job is very stressful and he’s glad that he can take the time to relax when he visits, it can’t be good for him to be so overworked. He just wants Theseus to be happy.
Albus is very, very careful. He knows he has to get this exactly right. Newt was young and pliable, but Theseus is a trained auror. If he suspects something is off Albus could loose everything. It’s risky, but after a lot of debate Albus decides it’s the best way.
He starts Theseus off on very small doses. Tiny amounts each time he visits which don’t do more than lift his mood slightly and make the visits particularly enjoyable. Slowly, over the course of months, he starts to visit more and more often. He doesn’t know why he’s doing it, other than that he just really enjoys seeing Albus and Newt. He’s so much happier when he’s with them, and so miserable when he’s stuck in everyday life.
When he starts visiting regularly, every weekend, Albus knows it’s time to increase the dose. Newt is glad to see his brother so relaxed and happy.
Not too long after that Theseus does realize something’s wrong – but it’s too late. He craves Albus’ potion and his presence even though he knows it’s been forced upon him, and he can’t bring himself to fight or report it. He realizes the same thing must have been done to Newt, it explains so much, but though he knows he should be livid about it – would have been, just a few short months ago – it doesn’t feel so important anymore. Only the potion is important.
Albus quickly ramps up the dose, keeping Theseus thoroughly drugged up while he conditions him. It still takes work, but he can feel the resistance melting away.