Cimetidine

Stomach acid may facilitate iron absorption. H-2 blocker drugs reduce stomach acid and are associated with decreased dietary iron absorption.¹ People with ulcers may also be iron deficient due to blood loss and benefit from iron supplementation. Iron levels in the blood can be checked with lab tests.

Hydrochloric acid is needed to release vitamin B12 from food so it can be absorbed by the body. Cimetidine, which reduces stomach acid, may decrease the amount of vitamin B12 available for the body to absorb.² The vitamin B12 found in supplements is available to the body without the need for stomach acid. Lab tests can determine vitamin B12 levels in people.

Cimetidine may reduce vitamin D activation by the liver.³ Lab tests can measure activated vitamin D levels in the blood. Forms of vitamin D that do not require liver activation are available, but only by prescription.

In healthy volunteers, a Reference magnesium hydroxide/Reference aluminum hydroxide antacid, taken with cimetidine, decreased cimetidine absorption by 20 to 25%.⁴ People can avoid this interaction by taking cimetidine two hours before or after any aluminum/magnesium-containing antacids, including magnesium hydroxide found in some Reference vitamin/mineral supplements. However, the available studies do not clearly indicate if magnesium hydroxide was the problem and may not need to be avoided.

Caffeine is found in coffee, tea, soft drinks, chocolate, Reference guaraná (Paullinia cupana), nonprescription over-the-counter drug products, and supplement products containing caffeine or guaraná. Cimetidine may decrease the clearance of caffeine from the body, causing increased caffeine blood levels and unwanted actions.⁵ People taking cimetidine may choose to limit their caffeine intake to avoid problems. They should read food, beverage, drug, and supplement labels carefully for caffeine content.