NZFSA said that the levels of vitamin K in these products, designed to boost the bone health of consumers, could interfere with the efficacy of warfarin, a drug commonly taken by heart disease patients.
Fonterra's Anlene brand, marketed for bone health, is popular around Asia and in its home market, leading the firm to add vitamin K to its recent brand extension, launched in New Zealand in May.
Vitamin K has been shown in some studies to improve bone mineral density (BMD) and reduce fracture incidence.
But although Fonterra said it communicated widely with healthcare professionals about Anlene when it was launched, including giving them detailed information on the potential implications for their patients on warfarin, the food authorities said the labeling should be clearer on the risks.
"Fonterra did its own risk assessments and believed the risk to be low but when it came on the market and we did an independent assessment we found the risk to be enough to require a warning," a spokeswoman from the NZFSA told AP-Foodtechnology.com.
Vitamin K, naturally found in leafy green vegetables, is not allowed to be added to most foods, including milk, under New Zealand's food regulations but Fonterra had marketed its products as dietary supplements.
However the NZFSA said that as the vitamin is not normally found in dairy products, people on anti-coagulation medication may not look out for its presence, while they would be more used to checking for vitamin K in multivitamins or other pills or capsules.
Anlene is the first dairy product with added vitamin K to launch on the New Zealand market, and a single serve contains about 40mcg of vitamin K1, similar to about 110mcg in half a cup of boiled broccoli.
There are some meal replacement products on the market with added vitamin K, such as Complan, which contains about half the amount.
But the NZFSA spokeswoman said that adding the vitamin to staple foods is 'an area of concern', suggesting future barriers to the marketing of foods fortified with the vitamin.
Fonterra has already launched the product in powdered format in Singapore and Malaysia last year, with no concern from authorities there.
But it has agreed to place a warning statement on the label of all Anlene products in New Zealand informing those taking the blood thinning medication Warfarin not to consume them without consulting a doctor.
A spokesman for the firm said that sales of the product to date are "tracking above expectations and we don't expect the warnings to affect sales in the future as less than 1% of New Zealanders consumers are prescribed warfarin-containing medication long-term" .
He added: "It is standard practice for these patients to be educated on the many dietary interactions that can occur with warfarin, including vitamin K." "No other products enriched with vitamin K carry warnings and vitamin K is also present in foods such as broccoli and spinach at much higher levels than Anlene," said the spokesman.