The explosive that became a heart medicineS

Nitroglycerin destroys lives by exploding, and saves them by stopping heart problems. How can it do both?

Most moviegoers have watched a scene or two where nitroglycerin has been used to blow the doors off of a bank vault. They've also seen scenes in which a character with chest pains is urged to take their nitroglycerin. Hollywood science isn't real science, but nitroglycerin is used for both healing hearts and cracking vaults. This doesn't seem to make sense. If nitroglycerin is an explosive, why don't people's hearts explode out of their ribcages like a chestburster when they take it?

For one thing, the substance comes in two different forms; solid and liquid. Nitroglycerin in either form is a wad of carbon, nitrogen, hydrogen and oxygen. In solid form it's stable. In liquid form, it is emphatically not. When it is in this form, it doesn't even need heat, the way other explosive substances do. A good physical knock will get it to tumble over like blocks, turning into carbon dioxide and water, with some nitrogen and oxygen gas floating around. During its tumble, it releases a lot of energy, which is where the bang comes from.

Nitroglycerin in solid form is a lot more stable. So stable that it was possible to include it in the mass manufacturing of dynamite. It was during that process that factory workers noticed a strange thing. Whenever they came into the factories, those with pains in their chest felt those pains subside. (This being the eighteen hundreds, any humanitarian consideration for factory workers was strictly forbidden – so the factory owners followed lopped off their noses or followed them home and beat their children in return for the free healthcare.)

The explosive that became a heart medicineS

In pill form, nitroglycerin is no longer one good shake away from blowing itself apart. The body, clever little devil that it is, shifts the nitroglycerin molecules around gently until they form nitric oxide. Nitric oxide is a kind of muscle relaxant, and the body is filled with muscles. Even blood vessels are surrounded by muscles, and when these muscles flex, they constrict the vessels enough that either less blood reaches its destination or the heart works harder, or both. Nitric oxide makes those muscles relax, and lets blood flow easily through the body. And there is no bang involved.

Via How Stuff Works and Straight Dope.