When you get a shot of vitamin B12, you're also getting traces of cyanide in the mix — although it would be equally accurate to say that there's artificial B12 in your antidote to cyanide.
A weird trick of biochemistry causes a chemical to catch and release cyanide, depending on the conditions. So you will probably take a little cyanide with your B12, but you can also take B12 to suck up cyanide.
Nothing you can see with the naked eye can manufacture vitamin B12 from scratch — which is a shame, because we all need it. Instead, we have bacteria to thank for keeping us alive: They can form B12, and we either eat them or eat things that eat them. Sometimes, we don't have the ability to absorb the vitamin B12 going through our guts, or just don't have the components in our body needed to absorb it into our system. This results in pernicious anemia, which starts with fatigue and depression and can get bad enough to bring on deterioration of the spinal cord and a heart murmur.
Doctors treat the condition with what they say is a vitamin B12 shot. Technically, that's correct. Cyanocobalamin is a form of vitamin B12 which our bodies can convert into the same B12 we'd pick up from food. One step in that process is changing cyanocobalamin to hydroxocobalamin. The body does this by wrenching away a cyanide molecule from cyanocobalamin, and letting it loose in the body.
Don't panic. The body can deal with small amounts of cyanide, usually by using sodium thiosulfate, converting the cyanide ions to thiocyanate, which you then excrete. Cyanocobalamin is safe, and found in food additives, health supplements, and the injections that doctors give patients with anemia.
What's more, the reaction can work the other way. Hydroxocobalamin is sometimes used to treat cyanide poisoning. When it gets into the blood stream, it can absorb cyanide atoms in the body.
So, depending on your condition, your doctor might either inject you with something that releases cyanide into your body — or grabs cyanide out of your body.