DNP is a class of chemical called an “uncoupler of oxidative phosphorylation". This means that DNP prevents most of the energy in nutrients from being used to make ATP (the fuel of life) and that instead, the nutrient energy gets burned off as heat. In fact, people who overdose on DNP literally cook themselves to death.
Very little of this was appreciated in 1933 when Cutting and Tainter--both medical researchers at Stanford University--published the first report on the use of DNP for weight control. Initially, DNP looked very promising, all the more so because it was incredibly effective. By 1934, at least 100,000 Americans were taking DNP for weight loss. Within three years after that, it became clear that DNP was pretty dangerous stuff. A number of people died by accidental overdose (as described above, they cooked from the inside-out) and hundreds of women who had taken DNP had rapidly developed severe cataracts that left them totally blind. By 1939, the FDA was extremely concerned about DNP and was finally able to ban it from all human use in the US. However, the story doesn't end there.
In the 1980's a Physician in Texas began using DNP on patients as part of an $11,000 weight loss program. He was ordering the chemical from Eastman Kodak Company who sold it only for use in film developing. The doctor would then have the chemical repackaged into a capsule and dispensed to patients. The doctor did not hide that he was using DNP-every bottle said so clearly- and patients lined up to get the stuff. As soon as the FDA learned of this, it wrote the doctor advising him that DNP was dangerous and not approved for human use. The doctor replied that the FDA had no authority over the practice of medicine (and to this day it does not) and that because he was only dispensing the chemical in one State, he was violating no federal law. It took several years for the State of Texas to force the doctor to stop prescribing DNP to people. Oddly the doctor did not go to jail for DNP, but he was sentenced several years later for insurance fraud.
Some hard-core bodybuilders still use DNP to burn fat and the chemical is widely advertised for sale on the internet. Because DNP is still being used by people, people are still dying from it too.