In addition to sulfur mustard and sesqui mustard, as mentioned above, there also exists nitrogen mustard. Unlike the former two, nitrogen mustard is still extensively produced - and widely used - today. However, instead of being used to injure and destroy, it is instead used to save lives from the ravages of cancer.

This drug, known more frequently as Mustargen (the trade name), mustine or mechlorethamine, was the first modern chemotherapy agent to be developed (though a very old drug, arsenic trioxide, has recently proven useful against some cancers). The compound was invented very much on purpose, as a chemical warfare agent, but its anti-cancer properties were discovered quite by accident. A stock of nitrogen mustard stored in Bari, Italy, was blown up during the second world war, exposing many civilians and US soldiers to the gas. The doctors treating them noticed that the victims' white blood cell counts were radically depressed. Normally, this myelosuppression would be a very bad thing - it led to infections, which often claimed the lives of mustard gas victims. But at least one doctor saw an interesting possibility.

Leukemia is a disease characterized by proliferation of abnormal white blood cells. The doctors treating the nitrogen mustard exposure victims reasoned that if nitrogen mustard could kill off normal white blood cells, perhaps it could kill the more rapidly dividing abnormal ones. So, a few daring oncologists tried it, injecting dying leukemia patients with deadly nitrogen mustard. Its side effects were many, and horrible - patients suffered debilitating nausea and uncontrollable vomiting, fever and chills, and their white blood counts plummeted - but the leukemia abated! A few patients even went into remission. Thus was born the first true hope against non-solid cancers.

Later, in the 1960s, nitrogen mustard was tried, along with the new drugs vincristine and procarbazine, against Hodgkin's Lymphoma, with resounding success. Not only did tumors shrink, but about 70% of the time, they never came back. Patients were actually cured of a disease that just a few years earlier had been a death sentence. Biochemists continued to experiment, and devised a whole host of mustard-based drugs. Among them are cyclophosphamide, instrumental in beating breast cancer, carmustine, one of the few drugs that can damage brain tumors, and melphalan, which is used in conditioning for bone marrow transplants. Even today, nitrogen mustard is still used to fight cancer, albeit much less often than before. Revolutions in side effect management have also made it much more tolerable than in the early days.

So there you have it - saving lives through chemical warfare. Who'da thunk it?