Second medical use means that chemical compounds or cells that are already used as active ingredients of medicine are used for a group of patients that are clearly different from the former group of patients or used for a region that is different from the former region of usage.
For example, Viagra was invented for treatment of a heart disease (heart attack) and later turned in to an ED medicine. Inventions based on second medical use of known chemical substances are eligible for patents, not only in Japan, but in several countries including America, Europe, Korea and China. When entering national phase in Japan with a European patent it is important to pay attention to the following. Usually in Europe inventions based on second medical use utilize Swiss type claims （"Use of a substance X for the manufacture of a medicament for the treatment of disease Y"）. However, if these claims are directly translated to Japanese, there is the problem that the claims refer to usage for manufacture, so the patent right might not be strong enough to oppose imported products or sales of the substance. Therefore in Japan, it is necessary to translate and amend the claims to turn the invention into an invention of a physical object (Invention of use).
Furthermore, since Swiss type claims have been abolished in Europe, future claims will turn into "Substance X for use in the treatment of disease Y” and thereby will be the same as claims for usage in Japan, therefore when entering national phase there will be less points to be cautious of.
However, in China, Swiss-type claims are currently the only way of validly claiming second medical uses of known substances. In Korea Swiss-type claims are not allowed and therefore they must be amended to claims directed to a pharmaceutical composition. (“ Substance treating disease Y comprising X as a main ingredient ”)