biologically active vitamin B12 when a microbiological assay with Lactobacillus delbrueckii subsp. lactis is used, as the bacteria can utilize the pseudovitamin despite it being unavailable to humans. To get a reliable reading of B12 content, more advanced techniques are available. One such technique involves pre-separation by silica gel and then assessment with B12-dependent E. coli bacteria. A related concept is antivitamin B12, compounds (often synthetic B12 analogues) that not only have no vitamin action, but also actively interfere with the activity of true vitamin B12. The design of these compounds mainly involved replacement of the metal ion. These compounds have the potential to be used for analyzing B12 utilization pathways or even attacking B12-dependent pathogens. Drug interactions H2-receptor antagonists and proton-pump inhibitors Gastric acid is needed to release vitamin B12 from protein for absorption. Reduced secretion of gastric acid and pepsin, from the use of H2 blocker or proton-pump inhibitor (PPI) drugs, can reduce absorption of protein-bound (dietary) vitamin B12, although not of supplemental vitamin B12. H2-receptor antagonist examples include cimetidine, famotidine, nizatidine, and ranitidine. PPIs examples include omeprazole, lansoprazole, rabeprazole, pantoprazole, and esomeprazole. Clinically significant vitamin B12 deficiency and megaloblastic anemia are unlikely, unless these drug therapies are prolonged for two or more years, or if in addition the person's dietary intake is below recommended levels. Symptomatic vitamin deficiency is more likely if the person is rendered achlorhydric (a complete absence of gastric acid secretion), which occurs more frequently with proton pump inhibitors than H2 blockers. Metformin Reduced serum levels of vitamin B12 occur in up to 30% of people taking long-term anti-diabetic metformin. Deficiency does not develop if dietary intake of vitamin B12 is adequate or prophylactic B12 supplementation is given. If the deficiency is detected, metformin can be continued while the deficiency is corrected with B12 supplements. Other drugs Certain medications can decrease the absorption of orally consumed vitamin B12, including: colchicine, extended-release potassium products, and antibiotics such as gentamicin, neomycin and tobramycin. Anti-seizure medications phenobarbital, pregabalin, primidone and topiramate are associated with lower than normal serum vitamin concentration. However, serum levels were higher in people prescribed valproate. In addition, certain drugs may interfere with laboratory tests for the vitamin, such as amoxicillin, erythromycin, methotrexate and pyrimethamine. Chemistry Vitamin B12 is the most chemically complex of all the vitamins. The structure of B12 is based on a corrin ring, which is similar to the porphyrin ring