B-Complex Injection

B-complex injections combine the water-soluble B-vitamins — typically B1 (thiamine), B2 (riboflavin), B3 (niacinamide), B5 (pantothenic acid), B6 (pyridoxine), and B12 (cobalamin) — into a single intramuscular or subcutaneous formulation.¹ The rationale is twofold: B-vitamin deficiencies often co-occur (shared dietary sources, shared absorption pathways), and injection bypasses GI uptake limits.

What the evidence supports

B-complex is well-established for repletion when one or more component vitamins is deficient. Documented deficiency is most common in: alcohol use disorder (thiamine), chronic GI disease, post-bariatric surgery, strict plant-based diets without supplementation, and certain chronic medications (metformin, PPIs, isoniazid).²³ In these populations, repletion produces measurable clinical improvement.

What the evidence does not support

In normo-replete patients — meaning men with normal labs and a reasonable diet — B-complex supplementation does not reliably improve energy, cognition, or athletic performance.⁴ The "wellness shot" marketing of B-complex is well ahead of the published data for healthy adults. Honest practice is to check labs first and treat deficiency when found, not to assume B-vitamins will fix non-specific fatigue.

Who benefits

• Documented deficiency in one or more B-vitamins.
• High alcohol intake (thiamine repletion is real medicine here).
• Post-bariatric surgery patients during the early post-op repletion phase.
• Strict plant-based diets without comprehensive supplementation.
• Patients on long-term metformin, PPIs, or isoniazid with documented deficiency.

Side effects are typically minor: injection-site soreness, transient bright-yellow urine from riboflavin excretion, occasional flushing. Niacin (B3) at higher doses can cause cutaneous flushing — most modern B-complex formulations use niacinamide to avoid this.