What is Methylcobalamin?
Methylcobalamin is a form and physiological equivalent of Vitamin B12. Methylcobalamin is an excellent source of Vitamin B12 as it is one of the only forms that are innately enzymatic cofactors; most other forms of B12 must convert first. 
Cofactors are molecules that help during biochemical transformations. Since Vitamin B12 is an essential nutrient, supporting its metabolism is crucial to obtain the most benefits and avoid the side effects of Vitamin B12 deficiency. 
Methylcobalamin vs. Cyanocobalamin
The basic structure of Vitamin B12 is Cobalamin. Various forms of this structure provide different benefits. Two common types of Vitamin B12 available on the market are Cyanocobalamin and Methylcobalamin.
Cyanocobalamin is made via bacterial fermentation with the addition of potassium cyanide. Once consumed, an adenosyl or methyl group replaces the cyanide ligand. Your body excretes the cyanide.  Methylcobalamin is Cobalamin combined with a methyl group.  With this form of Vitamin B12, your body does not need to process or excrete cyanide.
By taking Methylcobalamin, you skip a step in metabolizing the Vitamin B12. This is because Methylcobalamin is an active form of Vitamin B12 whereas Cyanocobalamin is not. Skipping this methylation step allows you to see benefits faster.
Adequate levels of Vitamin B12 are vital to numerous bodily functions. Individuals deficient in Vitamin B12 can experience cognitive impairments, a problem that worsens with age.  Vegetarians and vegans, in particular, are prone to Vitamin B12 deficiency as meat is a primary source of Vitamin B12.  By taking Methylcobalamin, individuals can maintain healthy levels of Vitamin B12 and help maintain their cognitive function.
Low levels of Vitamin B12 can also affect mood. Individuals suffering from poor moods often have insufficient levels of Vitamin B12.  One study found that by elevating Vitamin B12 levels, individuals were very likely to see an improvement in mood. 
Vitamin B12 is necessary to maintain energy levels. Individuals lacking in Vitamin B12 often experience fatigue and weakness.  Maintaining proper levels of Vitamin B12 helps individuals avoid this problem.
Individuals should take 100mg of Methylcobalamin 1%, which is the equivalent of 1mg Methylcobalamin, per day.