Nitrogen mustards are similar to sulfur mustards, but they have nitrogen in them instead of sulfur. While some countries stockpiled (gathered a lot of) nitrogen mustards during World War II, nitrogen mustards have never been used as chemical weapons. In fact, today, they are mainly used to treat medical problems. A few nitrogen mustards are too poisonous to be used for anything but chemical weapons. But many others are used as chemotherapy medications to treat cancer.