Taking large doses of vitamins B6 and B12 for an average 20 years is associated with increased risk of hip fractures (JAMA Netw Open, May 3, 2019;2(5):e193591). It is well-known that taking large doses of the fat-soluble vitamins — A, D, E and K — can harm you (Ann Intern Med, 2005;142(1):37-46). Large doses of vitamin D have been shown to increase risk for bone fractures (JAMA, 2010;303(18):1815-1822), and large doses of vitamin A are associated with increased risk for lung cancer in smokers (Int J Cancer, 2010;127(1):172-184). You may also be harmed by large doses of the water-soluble B vitamins (folate, thiamine, riboflavin, niacin, pantothenic acid, biotin, vitamin B6 and vitamin B12) or vitamin C (Int J Cancer, 2010;127(1):172-184).
The water-soluble vitamins have been felt to be safe because excessive dosages can pass out in your urine. Doctors used to prescribe large doses of the water-soluble vitamins, pyridoxine (B6), niacin (B3), and cobalamin (B12), to lower high blood levels of homocysteine that are associated with increased risk for heart attacks (Nutr J, Jan 10, 2015;14:6) and certain cancers (Radiol Oncol, 2010 Jun; 44(2): 79-85). They don’t do that anymore because even though taking B vitamins lowers homocysteine blood levels, it has repeatedly failed to reduce the rate of heart attacks (PLoS One, 2014;9(9):e107060; JAMA, Apr 28, 2010;303(16):1603-9) or cancers (Arch Intern Med, 2010;170(18):1622-1631). High levels of homocysteine weaken bones (Clin Chem Lab Med, 2007;45(12):1621-1632), but lowering homocysteine blood levels with vitamins B3, B6, and B12 does not prevent bone fractures (JBMR Plus, 2018;2(5):295-303). Instead, it increases bone fracture risk (Am J Clin Nutr, 2014;100(6):1578-1586; J Bone Miner Res, 2017;32(10):1981-1989).
How Most Vitamins Work
Before your body can use the food that you eat, your body must convert the food into the chemicals that provide you with energy and build all the structures in your body. The water-soluble B vitamins are parts of enzymes that start chemical reactions that provide you with these substances. The B vitamins cause Chemical A to be converted to chemical B plus other chemicals C, D, E, and so forth. When you eat the vitamins in food, other enzymes that we may not even know about in the food break down the remaining chemicals so they do not accumulate in your body to possibly harm you. However, when you take vitamins in a pill, you may not get all the other enzymes that you would have gotten in food to prevent you from accumulating harmful products in your body. For a more detailed explanation of how homocysteine is affected by the B vitamins, see Do You Need Vitamin Pills?
possible explanations for increased risk for bone fractures from high doses of
the B vitamins:
• They may interfere with nerve conduction to increase risk for falling and breaking bones (Acta Neurol Scand, 1987;76(1):8-11).
• They may block the bone-protecting effects of the female hormone estrogen (J Biol Chem, 1992;267(6):3819-3824).
• They may contain an inactive form of B6 that blocks the active form of B6 (Toxicol In Vitro, 2017;44:206-212).
I believe that you should not take vitamin pills in excess of your daily requirements unless you have a proven deficiency of a vitamin, as evidenced by low blood levels of that vitamin or a specific condition that requires large doses of that vitamin.