In some cases, patents may be accumulated on a compound and on the active metabolite that produces the desired effect in the body. For instance, in the case of terfenadine, which had been sold for many years in the United Kingdom as an antihistamine, the patent holder obtained a further patent on the active metabolite and attempted to block competition in the market of terfenadine, after the patent for the latter had expired. This was deemed to be an unacceptable attempt to extend patent protection¹¹².

¹¹² See, e.g., Grubb, 1999, p. 212-213.