Vitamin K describes a group of lipophilic vitamins that exist naturally in two forms: vitamin K1 (phylloquinone, found in green plants) and vitamin K2 (a group of menaquinones synthesised by bacteria in the intestine). Vitamin K3 (or menadione) is a synthetic form of vitamin K without a side chain. To become active, menadione needs to undergo prenylation. Vitamins K1, K2 and K3 are metabolically activated in the liver to become co-factors in the activation of vitamin K-dependent proteins, which are important for normal blood coagulation, and normality of bones and arteries (Gla proteins). Acute toxicity of menadione or its derivatives is reached at levels exceeding the requirements by a factor of at least 1 000. Menadione sodium bisulphite (MSB) and menadione nicotinamide bisulphite (MNB) are safe for all animal species at practical use levels in feed. The use of MSB in water for drinking is likely to increase the exposure of target animals to chromium(VI). Therefore, the FEEDAP Panel has concerns about the safety of MSB when administered by this route. The use of MSB and MNB in animal nutrition does not give rise to safety concerns for consumers. MSB is an eye irritant; in the absence of adequate data, the additive should be considered as a skin sensitiser. In the absence of data, MNB should be considered as irritant to skin and eyes and as a skin sensitiser. Considering the high dusting potential of MSB and MNB, the absence of data on inhalation toxicity and the chromium(VI) content of dust, inhalation exposure resulting from handling of MSB and MNB could be hazardous. The use of MSB and MNB in animal nutrition does not pose a risk to the environment. MSB and MNB are regarded as effective sources of vitamin K in animal nutrition.