The duration of the effectiveness of a B12 injection varies based on several factors, including the individual’s overall health, the severity of their deficiency, and the specific formulation of the B12 used. Generally, B12 injections can provide benefits that last from a few days to several weeks.

For individuals with severe deficiencies or specific medical conditions like pernicious anemia, the initial course of treatment often involves more frequent injections. These individuals might receive injections daily or weekly for several weeks to quickly build up their B12 levels. Once their levels are normalized, maintenance injections are typically administered every one to three months.

The body stores excess B12 in the liver, which can be used gradually over time. In healthy individuals with normal B12 levels, these stores can last for several years. However, for those who require regular injections due to absorption issues or ongoing medical conditions, the frequency of injections ensures that their B12 levels remain within a healthy range.

The formulation of the B12 injection also plays a role. Hydroxocobalamin, a common form used in injections, is stored longer in the body compared to cyanocobalamin, another form. Therefore, injections of hydroxocobalamin may be needed less frequently.

Symptoms of B12 deficiency, such as fatigue, weakness, or neurological issues, may begin to improve within a few days to weeks after the initial injection. Regular monitoring by a healthcare provider is crucial to determine the appropriate frequency of injections based on individual needs and response to treatment.

In summary, while the effects of a B12 injection can last from a few days to several weeks, the exact duration depends on individual health factors and the type of B12 used. Regular injections are often necessary to maintain optimal levels and prevent deficiency.