What do treatment drugs do to your brain?

My granddad was diagnosed with cancer, earlier this year. (prostate and then bone) After getting injections in the stomach every month ( not sure what the drugs are) he really started to pick up and look like his normal self. Over the last 2 months he has started to loose weight again and spend more time in bed again. After his injections he seems to turn very nasty. My nan is his full time carer and she gets the brunt of his nastiness, saying she's a terrible carer and he'd be better off without her, which has absolutely devastated my nan, (because she is and always has even before he was ill, waited on my granddad hand and foot) and she has broken down a couple of times. My nan has noticed a pattern of just after his monthly injections that he turns nasty...is this a normal reaction to the drugs? He is also hallucinating abit, seeing people who aren't there.
This is the first close relative who I have know to have cancer so its a shock, especially with someone who has never been seriously ill in their whole life.....