I'm 7 months into a one year course of Neratinib to prevent recurrence of breast cancer. When I was offered it, I searched  the forums for information and there was very little out there apart from a few horror stories of chronic diarrhoea as the main side effect. So I wanted to share my experience to reassure people that it's really not that bad. Of course, everyone will have a different response  

I started on 3 Imodium a day with the 6 Neratinib tablets, and the pharmacist said I could take up to 6. I'm now down to 1 or 2 Imodium a day. I can go for about 4 days with just one a day, but bowel movements get a bit soft and I get VERY gassy!! So I go back up to 2. So you do need to accept that the Imodium is essential with this treatment. 
However, contrary to some other posts, I've never had an accident or a time when I couldn't get off the loo, or had diarrhoea in my sleep as one poor poster said. I even took a week off work at the start expecting to be running to the loo but it didn't happen. I did get tummy ache, and at times, quite painful stomach cramps. But I haven't had any for a few months. I was also quite constipated at the start due to the Imodium working too well! 
I'm back working my full hours as a primary teacher and haven't needed to run to the loo once at work. Just begin to get a bit gassy late afternoon, especially if I've had something with lots of fibre. And I will have the odd loose bowel movement if I eat loads of fruit and veg and beans, but it's not a big  deal, and will be at my normal toilet time rather than an emergency toilet run.  At the start I didn't fancy fruit as it felt a bit sharp on the tummy, so be gentle with yourself. Now I eat anything I like. 
So if you're worried about starting Neratinib, give it a go, and hopefully, your experience will be a positive one too. 
Good luck, friends.