Pain-killing drugs (analgesic's) as the name suggests reduce pain. Most of the "over the counter" analgesic drug preperations contain aspirin, paracetamol, or combinations of both these substances with other ingredients.
From their molecular structures, it can be seen that paracetamol and the related compound phenacetin are derivitives of acetanalide; thier chemical names are p-hydroxyacetanalide and p-ethoxyacetanalide respecyively.
Acetanalide was once a popular drug marketed under the name antifibrin.
Phenacetin is no longer approved for medicinal use beacuse it is rather toxic, causing haemolytic anaemia or kidney damage in some patients. It was voted as a carcinogen by the environental protection agency in 1981.
Acetanalide causes a serious form of anaemia called methemoglobinaemia in which haemoglobin molecules in the blood stream are altered in such a way that their abilty to transport oxygen is reduced.
Even though antifibrin (acetanalide) is now considered too toxic for medical use, it's discovery did stimulate the development of safer and more effective analgesic drugs.
The discovery of antifebrin came about in 1886 when a pair of clinical assistants Arnold Cahn and Paul Hepp, were looking for somethign that would rid their patients of a particularly unpleasant intestinal worm. The trick wasto find a drug that would kill the worm but not the patient, their method was to test chemicals in the stockroom until they found one that worked. When someone came up with an ancient bottl labelled "napthalene", they tried it out on a patient who had every problem in the book, including the intestinal worms. It didn't kill the worms but it did reduce the patient's fever dramatically. Luckily before going out on a limb and endorsing napthalene as a cure for all fevers , Cahn and Hepp noticed that the chemical didn't have the characteristc "mothbal" odour, infact it was almost odourless, and so they decided to have it tested. Hepp's cousin a chemist at a near by dye factory, soon gave them the news that their new drug wasn't napthalene like they had assumed, but was infact acetanalide.
Barely 6 months after the discovery of antifibrin, a simmilar drug was develped. It was develped as a result of a storage problem. Carl Duisberg, director of research for the Friedrich bayer company, had to get rid of 50 tons of para-aminophenol - a seemingly useless yellow powder which was a by-product of dye manufacture. Rather than pay to have the stuff hauled away and dumped, Duisberg decided to turn it into something bayer could sell. after reading about antifibrin, he reasoned that a compound with simillar molecular structure might have the same therapeutic uses. Duisberg knew that a hydroxyl(OH) group attached to a benzene ring is a characteristc of many toxic substances (e.g. phenol), so he decided to 'mask' the phenol group in the para-aminophenol with an ethyl (CH3CH2) substituent. subsequent addition of an acetyl (CH3O) group produced phenacetin, which proved to be a remarkably effective and inexpensive analgesic.
Ironically, the substance Duisberg would have obtained if he hadn't masked the hydroxyl group is paracetamol which turned out to be a safer drug than phenacetin.
Paracetamol is now as popular as aspirin for treating fevers, headaches and other aches and pains. In the body acetanalide and phenacetin are converted to paracetamol, which is believed to be the active form of all 3 drugs.