Constipation, bloating, and gas are sometimes reported with the use of calcium supplements.¹¹ A very high intake of calcium from dairy products combined with large amounts of supplemental calcium carbonate (used as an antacid) 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) typically does not cause side effects. People with prostate cancer should avoid supplementing with calcium without medical supervision.

A combined analysis of 15 controlled trials found that long-term calcium supplementation was associated with a significant increase of approximately 30% in the incidence of myocardial infarctions (heart attacks).¹² Since these studies were not designed to examine the effect of calcium on heart attack risk, it is possible that the findings in this post hoc (after the fact) analysis were due to chance. A more recent study found that long-term calcium supplementation did not result in an increased incidence of cardiovascular disease-related death or hospitalisation.¹³ Moreover, a pooled analysis of randomised controlled trials found that supplementing elderly individuals with a combination of calcium and vitamin D significantly decreased the mortality rate by 7%.¹⁴

In the past, calcium supplements in the forms of bone meal (including microcrystalline hydroxyapatite [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.¹⁵ “Refined” forms (which would include calcium citrate malate [CCM], calcium citrate, and most calcium carbonate) have low levels of lead.¹⁶ More recently, a survey of over-the-counter calcium supplements found low or undetectable levels of lead in most products,¹⁷ 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.