Are there any side effects or interactions?

Constipation, bloating, and gas are sometimes reported with the use of calcium supplements.⁶ A very high intake of calcium from dairy products plus supplemental calcium carbonate was reported in the past to cause a condition called “milk alkali syndrome.” This toxicity is rarely reported today because most medical doctors no longer tell people with ulcers to use this approach as treatment for their condition.

People with hyperparathyroidism, chronic kidney disease, or kidney stones should not supplement with calcium without consulting a physician. For other adults, the highest amount typically suggested by doctors (1,200 mg per day) is considered quite safe. People with prostate cancer should avoid supplementing with calcium.

In the past, calcium supplements in the forms of bone meal (including MCHC), dolomite, and oyster shell have sometimes had higher lead levels than permitted by stringent California regulations, though generally less than the levels set by the federal government.⁷ “Refined” forms (which would include CCM, calcium citrate, and most calcium carbonate) have low levels.⁸ More recently, a survey of over-the-counter calcium supplements found low or undetectable levels of lead in most products,⁹ representing a sharp decline in lead content of calcium supplements since 1993. People who decide to take bone meal, dolomite, oyster shell, or coral calcium for long periods of time can contact the supplying supplement company to request independent laboratory analysis showing minimal lead levels.

Some studies have shown that calcium competes for absorption with a number of other minerals, while other studies have found no such competition. To be on the safe side, some doctors recommend that people taking calcium for long periods of time should also take a multimineral supplement.

One study has shown that taking calcium can interfere with the absorption of phosphorus, which, like calcium, is important for bone health.¹⁰ . Although most western diets contain ample or even excessive amounts of phosphorus, older people who supplement with large amounts of calcium may be at risk of developing phosphorus deficiency. For this reason, the authors of this study recommend that, for elderly people, at least some of the supplemental calcium be taken in the form of tricalcium phosphate or some other phosphorus-containing preparation.

Vitamin D’s most important role is maintaining blood levels of calcium. Therefore, many doctors recommend that those supplementing with calcium also supplement with 400 IU of vitamin D per day.

Animal studies have shown that essential fatty acids (EFAs) increase calcium absorption from the gut, in part by enhancing the effects of vitamin D and reducing loss of calcium in the urine.¹¹

Lysine supplementation increases the absorption of calcium and may reduce its excretion.¹² As a result, some researchers believe that lysine may eventually be shown to have a role in the prevention and treatment of osteoporosis.¹³

Are there any drug interactions?
Certain medicines may interact with calcium. Refer to drug interactions for a list of those medicines.