There was a movie about a woman who marries a man. And slowly finds out something is wrong. He slowly starts drugging her to make it look like she is going insane so he can be sole custody over her money. He is in this plan with I think her step-sister or someone close to her. It is a suspense. She slowly starts figuring out what is going on but by that time it is too late because no one will believe her now because they think her state of mind is compromised. So when she starts to question and become unreasonable he almost kills her. They are in the bathroom and he cuts her and hits her and everything and then she wakes up in the hospital and she knows he did it but everyone thinks she did it to herself. So she tried to figure out what is going on to prove. She finds the drugs in the bedroom drawer and knows. She slips some into his wine and relizes when he is passed out cold she starts hitting him and slapping him and swearing and then collects herself and tries to find out as much as possible before he wakes up. He realizes she found out and is going to try to kill her so she is trying to get someone to believe her before it is too late. She ends up living and proving she was not insane and that bad people were after her. I can't remember the dern name or actors/actresses any idea?

There was a movie about a woman who marries a man. And slowly finds out something is wrong. He slowly starts drugging her to make it look like she is going insane so he can be sole custody over her money. He is in this plan with I think her step-sister or someone close to her. It is a suspense. She slowly starts figuring out what is going on but by that time it is too late because no one will believe her now because they think her state of mind is compromised. So when she starts to question and become unreasonable he almost kills her. They are in the bathroom and he cuts her and hits her and everything and then she wakes up in the hospital and she knows he did it but everyone thinks she did it to herself. So she tried to figure out what is going on to prove. She finds the drugs in the bedroom drawer and knows. She slips some into his wine and relizes when he is passed out cold she starts hitting him and slapping him and swearing and then collects herself and tries to find out as much as possible before he wakes up. He realizes she found out and is going to try to kill her so she is trying to get someone to believe her before it is too late. She ends up living and proving she was not insane and that bad people were after her. I can't remember the dern name or actors/actresses any idea?

It's not Gas Light but it is very similar it's more modern though.

Is Cary Grant in it?

This movie is similar to what you're describing (doesn't fit perfect - but I keep going back to it... ) 'House of Secrets'... read the second review on this link, it gives a pretty good synopsis. Let me know if this is what you're looking for!
House of Secrets Reviews & Ratings - IMDb (http://www.imdb.com/title/tt0107150/reviews)