Green tea has polyphenols, primarily epicatechin and epicatechin gallate which can decrease the effectiveness of TKI's such as nilotinib (Tasigna).
But it is important to note that green tea polyphenols metabolize quickly in the body (a few hours) and that the half-life of nilotinib is similar (4-5 hours). It is best to avoid taking nilotinib at the same time as green tea. But I would not avoid drinking green tea altogether as its benefits are clear. Separating in time by six hours when you take nilotinib and drink green tea should easily avoid drug-tea interaction.
In my own case, I drank and continue to drink green tea all day long (after morning coffee). I took dasatinib (back in the day, years ago now) at night before sleep, so many hours separated green tea consumption from TKI ingestion. Nilotinib is typically taken twice a day. As long as you have 4 or so hours separating the two compounds, interaction will be minimal. I would not avoid green tea. It is good for you.
(Imatinib, in contrast with dasatinib and nilotinib, has a much longer half-life at 18 hours. And imatinib is more dose dependent (more works better) than the other TKI's. I would avoid green tea ingestion while taking imatinib. Switching to dasatinib, for example, would eliminate this issue)
