I am a vegan living in India where the B12 supplements available only contain methylcobalamin and I am taking those. But I have read in Wikipedia and heard on YouTube that cyanocobalamin/adenosylcobalamin is the preferred choice of supplements. What's up with this?
There's no need to worry about this.
Methylcobalamin is a natural form of B12 (found in animals including humans), whereas cyanocobalamin is a synthetic form which is cheaper to produce and considered to be slightly more stable than methylcobalamin (meaning can have a longer shelf life). Cyanocobalamin is converted into methylcobalamin or adenosylcobalamin inside the human body.
While there is some evidence that cyanocobalamin is slightly more readily absorbed from the gut than methylcobalamin, this is likely more than compensated for by the fact that cyanocobalamin is more readily excreted in urine than methylcobalamin, and so less well retained.
I haven't seen any supplements in the form of adenosylcobalamin, and I'm not aware of its specific benefits, if any. Adenosylcobalamin is a natural form of B12 found in the human body.
The bottom line here is that supplements containing methylcobalamin have been shown to reverse and prevent deficiency of vitamin B12, as have supplements containing cyanocobalamin. That is: they both do the job for us vegans and it really doesn't matter which one you take.*
You mentioned that you only find supplements containing methylcobalamin and not cyanocobalamin. I just searched online myself and I was able to find cyanocobalamin supplements for sale here in India, so I guess you can order them if you want to. Personally I don't give credence to the idea that cyanocobalamin is better - maybe for manufacturers and perhaps for your pocket it is, but from a health perspective I think methylcobalamin is at least as good.
* However, people with severe kidney dysfunction or who are taking certain medications could possibly have a risk of cyanide toxicity from taking high doses of cyanocobalamin. Please check with a doctor for personal medical advice.
Methylcobalamin is probably the best form one can obtain in a supplement. It usually costs considerably more, but the difference is well worth it. Studies have shown that 98% or more of cyanocobalamin exits the body within about 24 hours--even when injected. Methylcobalamin has a much higher absorption rate.
Be aware, however, that some companies in Asia market a supplement under the name of "Mecobalamin." This makes one think it should be methylcobalamin, but, and assuming they knew the truth, the pharmacists informed me that it was actually hydroxocobalamin...tricky.
Some believe that adenosylcobalamin is the best form, but in reality, both it and methylcobalamin are used in the body. Cyanocobalamin must be processed and converted to another form in the body before it can be useful. Adenosylcobalamin is unlikely to be encountered in a supplement form.
Regardless of the form of B12, if one is deficient it is important to have a source of B12 every day. Essentially, the body is limited with how much it can absorb at one time. The limit is on account of the availability of intrinsic factor. Older individuals produce less intrinsic factor, and people with certain health conditions may produce less as well. Without intrinsic factor, B12 cannot be absorbed.
Vitamin B12 is the largest vitamin, size-wise. In order for it to pass through the lining of the intestine, the intestinal wall must open a special hole for it to pass through, then close the hole again afterward to prevent the entrance of bacteria. It is intrinsic factor that causes this opening. Because of this, one can consume ever so much B12 and not absorb any of it if the intrinsic factor is lacking. This is why B12 shots (injections) are a hundred times more effective at getting the B12 into the bloodstream.