Because various vitamins were mistakenly included in group B.
The letter designations for vitamins began to be used in the early 20th century when the possibilities for chemical analysis were quite limited. Vitamins were named not by their chemical composition but by their physiological effects. For the B vitamins, this effect is quite similar, and they were usually found in the same foods, which is why they were initially grouped under the name 'vitamin B.'
Later, it became clear that this is a whole group of substances with different chemical natures that are not interchangeable. As they were isolated, they were assigned indices B1, B2, and so on. It was later discovered that some of them lack vitamin properties, while others were discovered twice by different scientists, leading to them being assigned different indices.
After excluding such cases, there are only seven B vitamins left: B1, B2, B3, B5, B6, B9, and B12. However, in modern medical literature, the designations B6 and B12 are most commonly encountered, as each refers to several similar substances with the same physiological effects. The other B vitamins are usually designated by their chemical names: B1 – thiamine, B2 – riboflavin, and so on.
