If you are currently being treated with any of the following medications, you
should not take vitamin K without first talking to your healthcare provider.
Antibiotics
Antibiotics, particularly a class known as
cephalosporins, reduce the absorption of vitamin K. Extended use of antibiotics
may result in vitamin K deficiency because these drugs kill not only harmful
bacteria, but also beneficial, vitamin K-activating bacteria. This is
particularly a problem for people who already have low levels of vitamin K or
are at risk for deficiency (such as those who are malnourished, elderly, or
taking warfarin).
Doxorubicin
Preliminary evidence suggests that vitamin K3 (a
synthetic form of vitamin K) may enhance the chemotherapeutic effects of
doxorubicin. However, these results have not yet been demonstrated in people.
There is also some concern that vitamin K3 causes severe anemia in people. More
research is needed to assess if this synthetic form of vitamin K is safe and
useful.
Phenytoin
Phenytoin interferes with the body's ability to use
vitamin K. Taking anticonvulsants (such as phenytoin) during pregnancy or while
breastfeeding may deplete vitamin K in newborns.
Warfarin
Vitamin K reduces the effects of the blood-thinning
medication warfarin, rendering the medication ineffective. Vitamin K should not
be taken while taking warfarin, and foods containing high amounts of vitamin K
should be avoided.
Weight Loss Products
Orlistat, a medication used for weight
loss and olestra, a substance added to certain food products, are both intended
to bind to fat and prevent the absorption of fat and the associated calories.
Because of their effects on fat, orlistat and olestra may also prevent the
absorption of fat-soluble vitamins such as vitamin K. Given this concern and
possibility, the Food and Drug Administration (FDA) now requires that vitamin K
and other fat soluble vitamins (namely, A, D, and E) be added to food products
containing olestra. How well vitamin K from such food products is absorbed and
used by the body is not clear. In addition, physicians who prescribe orlistat
add a multivitamin with fat soluble vitamins to the regimen.
The possibility of olestra or orlistat interfering with vitamin K absorption
is particularly important to know if you already have a vitamin K deficiency
(such as with malnourishment or poor fat absorption from cystic fibrosis) or if
you have a tendency to bleed (includign if you take the blood thinner warfarin).
On the other hand, the fact that vitamin K is now added to olestra-containing
food products is also significant if you should not be taking vitamin K (again,
for example, if you are on the blood thinner warfarin or you have a G6PD
deficiency).
X-rays and Radiation
X-rays and radiation can deplete vitamin
K levels and raise vitamin K requirements.
Each of the following may diminish vitamin K absorption and lead to reduced
levels in the body:
- Aspirin
- Cholestyramine, one of a class of medications known as bile
acid sequestrants used to lower cholesterol
- Mineral oil laxatives
