Most vitamin B12 supplements contain cyanocobalamin, a form of the vitamin that contains small amounts of cyanide. Once you’ve ingested the supplement, your body removes and detoxifies the cyanide, which is there simply to stabilize the vitamin. The cobalamin is then converted to methylcobalamin, which is one of the active forms of vitamin B12.

All that detoxifying and converting sounds like a lot of extra work—so why not just take a supplement that contains cyanide-free, preformed methylcobalamin?

One reason is that methylcobalamin hasn’t been studied nearly as well as cyanocobalamin. Cyanocobalamin has been used extensively in studies to reverse vitamin B12 deficiency and it’s shown to be very effective. In contrast, methylcobalamin may produce a less stable supplement. It doesn’t mean it’s not effective, but it probably takes much greater amounts of methylcobalamin to achieve and maintain adequate vitamin B12 levels. The usual recommendation for adults is 25 to 100 micrograms of vitamin B12 as cyanocobalamin per day. If you’re taking methylcobalamin, you are likely to need 10 to 40 times as much—1,000 micrograms per day.

Some vegans are concerned about the fact that cyanocobalamin has to be converted to methylcobalamin. Does that extra step matter? Given the research on the efficacy of cyanocobalamin, it doesn’t seem to. In fact, methylcobalamin has to be converted, too. It’s able to perform some but not all of the functions of B12 and has to be converted to a compound called adenosylcobalamin to do the rest.

The amount of cyanide in a 1,000 microgram tablet of cyanocobalamin is far less than would affect your health. Although cyanocobalamin is not recommended for people with kidney disease (because healthy kidneys are needed to detoxify cyanide) or for people who have had cyanide poisoning, it’s safe for everyone else. For vegans, who depend on supplements or fortified foods to meet vitamin B12 needs, cyanocobalamin is probably the best choice. If you take methylcobalamin, be sure to get at least 1,000 micrograms per day. Look for chewable tablets to make sure you’re absorbing all of the B12 in your supplement.