The 77-year-old man was taken to hospital four times in the past two months - complaining of pain and bleeding, with symptoms similar to an overdose of blood-thinning drugs. A doctor took a blood test and realised he had been poisoned.
Police searched the couple's house and found a packet of half-empty rat poison in the kitchen pantry.
The man's wife wasn't a suspect at first, as she had brought her husband to hospital each time, but when police questioned her she confessed to poisoning him. She said she had stirred it into his coffee as she felt she couldn't cope any more and was desperate.
The couple have been married for 47 years. The woman will be held in police custody before her trial and will be examined by a psychiatrist.