We encountered a case of hypercobalaminemia induced by oral intake of an energy drink after total gastrectomy. The patient was referred to our hospital due to findings suspicious for gastric cancer on screening. A 20 mm type 0-IIc lesion was detected in the gastric subcardia on esophagogastroduodenoscopy. Total gastrectomy followed by Roux-en-Y reconstruction was performed. He was discharged without complications. His basal serum vitamin B₁₂ level was initially maintained with monthly intramuscular injections of vitamin B₁₂. After 9 months, his serum vitamin B₁₂ level suddenly increased up to 36-fold higher than the normal range and persisted there for one year without vitamin B₁₂ injections. The patient ultimately reported consuming half a bottle of an energy drink each day during this time period. This case demonstrates the risk of unexpected hypervitaminemia resulting from self-administration of nutritional supplements.