Vitamin K2, or menaquinone, may cause an allergic reaction in rare cases, according to WebMD. This supplement is not known to have other side effects. An allergy might occur from one of the inert ingredients in the pill rather than from the vitamin.
Doctors prescribe Vitamin K2 for patients who have osteoporosis or bone loss due to steroids, states WebMD. They also give it to patients undergoing dialysis to lower their cholesterol. Vitamin K in any form plays a role in blood clotting, so patients may take it to counteract the effects of blood thinners. Patients taking blood thinners or non-steroidal anti-inflammatory medicines, such as aspirin, should inform their doctors before taking Vitamin K2.