Slang term for Ibuprofen. The term "Vitamin I" is used amongst some athletes and outdoorspeople to refer to Ibuprofen, because of its necessity, ubiquity and (relative) harmlessness.
While ibuprofen is a very handy medication to have for minor aches and pains, and can be quite necessary for some accidents or illnesses, it is also, of course, a drug. Ibuprofen, while much less toxic than other drugs, should still be used sparingly, since it can cause hepatic and renal toxicity. So despite the cute nickname, do not treat ibuprofen like a vitamin.