I haven't taken neratinib, but I was on a different kinase inhibitor as part of a clinical trial. It caused HORRENDOUS diarrhea for me (8 Imodium and 3 Lomotil per day didn't really control it at its worst.) This was a surprise, because diarrhea isn't usually my problem. But apparently other people on the trial didn't have nearly as bad a problem. So these drugs seem to have different effects on different people.

I would be prepared to pretty much stay home for a month while you're figuring out how to control it. Also, I learned that if I hadn't taken enough Imodium the day before, there was nothing I could do that day to control it.

And for me, it took 9 days for the diarrhea to start, so it may take a while to see how it will be long-term.

Hopefully it won't be anywhere near that bad for you.