Just an interesting observation: People with known mental health conditions are twice as likely to poison themselves in a suicide than those without known mental health conditions.
Firearms are more popular with men and poison is more popular with women. (edit: source https://en.wikipedia.org/wiki/Gender_differences_in_suicide#...) The charts in the article show that women who commit suicide are more likely to have a known mental health condition than men, so that could explain the correlation.