When you take vitamin B12 in a supplement, the chances are that it contains cyanocobalamin. This is a form of B12, but one that is only produced in the lab - it's a synthetic, manmade alternative to the B12 that you find in food, methylcobalamin.
Cyanocobalamin gets preferential treatment from the supplement industry because it's cheaper to produce and store, but there's a growing body of evidence that methylcobalamin could well be the superior ingredient.
One of the most commonly cited arguments is that your body can't actually use cyanocobalamin straight away - it must first be converted into one of the bioavailable forms (generally either methylcobalamin or hydroxocobalamin). So why give your body something it can't use without conversion, when you can give it the final product from the start?
What's more, this internal conversion process involves replacing a cyanide group in the molecule. This means that by taking cyanocobalamin supplements, you're actively releasing small quantities of cyanide into your bloodstream - something that most people aren't very happy about when they find out!
The benefits of methylcobalamin over the alternatives don't stop there either. There's a possible link between methylcobalamin and improved visual accommodation, a link that hasn't been found for cyanocobalamin.
Some research has also found evidence that methylcobalamin may be stored for longer in the body, and at higher concentrations. So taking methylcobalamin supplements should actually reduce the need for further supplementation!
Vitamin B12 deficiency can be a very serious illness, and taking B12 supplements is naturally one of the most common treatments. But if you take a non-methylcobalamin form of the vitamin, then your body has to convert it before it can be used, and some people cannot do this effectively.
This means that while trying to treat a serious illness, using the inferior cyanocobalamin ingredient could hamper their recovery. Heavy smokers are a classic example of a group who may have trouble converting cyanocobalamin into methylcobalamin, due to the build up of heavy metals and toxins that accumulate in the liver.
So while cyanocobalamin supplements may be a little cheaper and there are more of them to choose from, it seems likely that your body will thank you for giving it the methylcobalamin it needs.