Short answer: Both are forms of vitamin B12. Methylcobalamin is one of the two forms the body uses directly; cyanocobalamin, the usual supplement form, works once the body converts it.1 No randomized trial has compared them head to head. Two look-back studies in children found sublingual methylcobalamin raised B12 levels about as well as cyanocobalamin pills or shots.23
| Methylcobalamin | Cyanocobalamin | |
|---|---|---|
| What it is | One of the two active forms of B12 in the body1 | Becomes active after the body converts it to methylcobalamin or adenosylcobalamin1 |
| In supplements | Used in some products1 | The form most often found in supplements1 |
| Studies in low B12 | Two retrospective studies in children (158 and 312): sublingual methylcobalamin raised levels as well as cyanocobalamin23 | Randomized dose-finding trial in 120 older adults: about 650 to 1,000 mcg a day gave most of the possible effect4 |
| Randomized head-to-head trial | None found in the sources checked | None found in the sources checked |
| Safety | No upper limit; NIH calls B12 generally safe even at high doses1 | Same1 |
NIH lists methylcobalamin and 5-deoxyadenosylcobalamin as the active forms of B12. Cyanocobalamin and hydroxocobalamin become active once the body turns them into one of those two. Supplements often use cyanocobalamin, though other forms are also sold.1 Some medicines, including metformin and stomach acid blockers, may affect B12 levels.1
Methylcobalamin. In a look-back study of 158 children aged 0 to 3 with low B12, levels rose above 300 ng/L with oral cyanocobalamin, sublingual methylcobalamin and cyanocobalamin injections alike.2 In another look-back study of 312 children, a sublingual methylcobalamin spray and cyanocobalamin injections gave similar B12 levels at 6 weeks and 3 months.3 Both studies changed the route as well as the form, and neither was randomized.
Cyanocobalamin. In a randomized trial in 120 older adults with mild deficiency, daily oral cyanocobalamin was tested at 2.5 to 1,000 mcg for 16 weeks. Doses of about 650 to 1,000 mcg gave 80% to 90% of the largest drop in methylmalonic acid, a blood marker of B12 shortage. That is more than 200 times the daily need.4 A Cochrane review found low-quality evidence that oral B12 at 1,000 mcg a day normalized blood levels about as well as injections; the abstract does not name the form.5
No randomized trial has tested one form against the other, so the claim that methylcobalamin works better rests on thin evidence. Both have raised B12 levels in people who were low. This is general information, not personal advice. More detail: the Vitamin B12 (Cobalamin) guide.