<unk>  is a suspension containing particles of the radioactive compound thorium dioxide, ThO2, used as a contrast medium in X-ray diagnostics in the 1930s and 40s (use in some countries, such as the U.S., continued into the 1950s).
The naturally abundant thorium nuclide 232Th is slightly unstable and decays through the emission of an alpha particle. Thorium compounds produce excellent images because of thorium's high opacity to X-rays (it has a high cross section for absorption).  Because the suspension offered high image quality and had virtually no immediate side-effects compared to the alternatives available at the time,<unk>  became widely used after its introduction in 1931. 2-10 million patients worldwide have been treated with<unk> .
Even at the time of introduction, there was concern about the safety of<unk> . Following injection, the drug is distributed to the liver, spleen, lymph nodes, and bone,where it is absorbed. After this initial absorption, redistribution takes place at a very slow pace.  Specifically, the biological half-life is estimated to be 22 years.  This means that the organs of patients who have been given<unk>  will be exposed to the alpha-radiation it emits for much of the rest of their lives. The significance of this exposure was not fully understood in 1931.
Unfortunately,<unk>  turned out to be extremely carcinogenic, because of these alpha particles. There is a high over-incidence of various cancers in patients who have been treated with<unk> . The cancers occur several (usually 20-30) years after injection of<unk> . The risk of getting liver cancer in former<unk>  patients has been measured to be well above 100 times the risk of the rest of the population. The risk of leukemia appears to be 20 times higher in<unk>  patients. These observations have led some to characterize<unk>  as the most potent human carcinogen known.
The Danish director Nils Malmros's movie, Facing the Truth (original Danish title At Kende Sandheden) from 2002, portrays the dilemma that faced Malmros's father, Richard Malmros, when treating his patients in the 1940s. Richard Malmros was deeply concerned about the persistence of<unk>  in the body but was forced to use<unk> , because the only available alternative (per-abrodil) had serious immediate side-effects, suffered from image quality problems and was difficult to obtain during the Second World War. The use of<unk>  in Denmark ended in 1947 when safer alternatives became available. Today, barium sulfate is the most commonly-used X-ray contrast medium.<unk>  has also been used in research to stain neural tissue samples for examination by historadiography.
Thorotrast
Iofendylate
Azelastine 
Pemirolast
Bitolterol
Bosentan
Indoramin
Acrivastine