Nearly constant pain causing hallucinations?

I was just wondering if someone who was in nearly constant pain (severe), could cause someone to eventually lose their mind? Someone told me about their grandfather who was a POW and he was tortured all the time. She told me it caused him to hallucinate, be out of touch with reality, and eventually just completely lose it. She said he died that way. I was just wondering if that can actually happen. She's been known to kind of stretch the truth sometimes, but I'm still curious if pain can cause someone to lose their mind, even if she was lying about her grandfather.