Abstract: Activated carbon (for example medicinal carbon), which has long been used for adsorbing and removing harmful substances from the digestive system, has the drawback of causing constipation, and the production of a spherical adsorbent carbon that is believed to overcome this drawback from a petroleum-based hydrocarbon requires a complicated production process. These issues can be addressed by an oral adsorbent, a therapeutic or prophylactic agent for kidney disease, and a therapeutic or prophylactic agent for liver disease, containing a fibrous carbonaceous substance obtained by carbonizing cotton fiber. The issues can also be addressed by an oral adsorbent, a therapeutic or prophylactic agent for kidney disease, and a therapeutic or prophylactic agent for liver disease, containing a fibrous carbonaceous substance having a twisted hollow ribbon-like structure.