Hormonal birth control: injection

What it is: A shot of progesterone.
How it works: Prevents ovulation and thins the lining of the uterus.  
Effectiveness: About 99.7%.
Convenience: You need to see your doctor every three months for the injection.
Best for: Women who can’t have estrogen, including nursing mothers.
Side effects: Irregular bleeding and weight gain of up to five pounds. About 50% of women stop having periods. It can take up to a year to become fertile again.
Price tag: May be covered by private insurance.
Good to know: Progesterone can lead to bone density loss, which improves when the injections are stopped.

Our experts:
Dara Maker, family physician at Women’s College Hospital in Toronto
Roey Malleson, clinical associate professor of family practice at University of British Columbia
Ronald Weiss, assistant professor of medicine at University of Ottawa