I have watched more than one documentary on television about wives who are poisoning their husbands. They nearly got away with it, too, because they were attempting it slowly over time.
One lady's husband kept being hospitalized because of severe cramps and vomiting. His health seemed to be dwindling away, but the doctors did not know why.
One nurse saw the lady putting something in her husband's food while he wasn't looking. Because of her suspicions, she tested his urine for arsenic. His wife got arrested after the test confirmed its presence.