Because calcium supplements may interfere with alendronate absorption, they should be taken two hours before or after the drug (PDR 1998).

In a study with eight male subjects, 5 mL of aluminum hydroxide gel (2.4 gm qid) coadministered with calcium citrate (950 mg qid) for three days increased urinary aluminum excretion (Coburn et al. 1991). The finding of enhanced aluminum excretion is consistent with another study involving 30 healthy women who were given calcium citrate (800 mg elemental calcium/day) (Nolan et al. 1994). Urinary and plasma aluminum levels were increased significantly. This effect may have been related to aluminum derived only from dietary sources because the women were not receiving aluminum-based antacids.

Two doses of amiloride (2.5 mg/day) reduced urinary calcium in subjects with kidney stones (Leppla et al. 1983). This decrease in calciuresis was enhanced when amiloride was coadministered with two doses of hydrochlorothiazide (25 mg/day).

Oral administration of 500 mg calcium salts (lactate, gluconate, and carbonate) with atenolol (100 mg) reduced plasma levels of atenolol by 51% in six healthy subjects (Kirch et al. 1981). Long-term coadministration increased the elimination half-life and led to atenolol accumulation. Subsequent studies did not confirm an interaction between atenolol and calcium antacids (Gugler and Allgayer 1990). Until more is known, individuals on beta-blockers should have their blood pressure checked before and after the addition of calcium antacids or supplements to their atenolol regimen.

Hypocalcemia can negate the therapeutic effects of digoxin, while hypercalcemia may predispose a patient to arrhythmias and digoxin toxicity (Hines Burnham et al. 2000). In one study, patients with digoxin-induced cardiotoxicity had serum concentrations of the drug that were within therapeutic range but they had higher calcium to potassium ratios (Sonnenblick et al. 1983). Normal levels of calcium should be maintained during digoxin treatment. Patients taking digoxin should have calcium blood levels monitored closely.

Conjugated estrogens lower calcium excretion (Lobo et al. 1985) and increase calcium absorption in postmenopausal women (Gallagher et al. 1980). The enhanced absorption appears to be due to an increase in serum 1,25(OH)2D. Early postmenopausal women taking calcium supplements with estradiol (or conjugated estrogens) have been shown to have significantly greater gains in bone mineral density than women taking HRT alone (Pines et al. 1999). Calcium supplementation is highly recommended in all postmenopausal women. For women 51 years or older, the Dietary Reference Intake (DRI) for calcium is 1200 mg/day (Institute of Medicine 1997).

In a retrospective study, coronary artery bypass graft patients who received both a bypass prime with a high calcium concentration and gentamicin perioperatively had a higher incidence of renal failure compared with those who received only the prime, gentamicin alone, or neither (Schneider et al. 1996). Concomitant administration of calcium may potentiate gentamicin-induced nephrotoxicity.

Quinolone antibiotics form chelates with metal cations, such as aluminum, magnesium, calcium, iron, zinc, copper, and manganese (Kara et al. 1991; Li et al. 1999), which significantly reduces the absorption of these medications (Balfour and Wiseman 1999; Brouwers 1992; Campbell and Hasinoff 1991). Dietary supplements and antacids containing aluminum and magnesium should be taken two to four hours before or after administration of these antibiotics (Hines Burnham et al. 2000; Li et al. 1999).

Tetracyclines form chelates with divalent and trivalent cations, including iron, aluminum, magnesium, and calcium (Neuvonen 1976). These chelates are poorly soluble and can significantly reduce the absorption and efficacy of tetracyclines (Hines Burnham et al. 2000; Neuvonen 1976). Calcium salts should be administered at least two hours before or after tetracyclines (Hines Burnham et al. 2000).

Thiazide diuretics may cause hypercalcemia by decreasing calcium excretion (Hines Burnham et al. 2000). Treatment with a combination of hydrochlorothiazide (50 mg/day) and vitamin D in six postmenopausal women with osteoporosis for six months reduced urinary calcium excretion by 22% (Sakhaee et al. 1984). In this study, it was noted that the combination of hydrochlorothiazide and vitamin D also decreased calcium absorption by 25%.

However, the ability of thiazide diuretics to decrease urinary calcium excretion has been associated with less risk of hip fractures in patients taking these medications (Rejnmark et al. 1998).

It has been reported that calcium salts may reverse the clinical effects and toxicities associated with verapamil (Hines Burnham et al. 2000). However, pretreatment with intravenous calcium in patients with supraventricular arrhythmias reduced the incidence of hypotensive side effects without compromising the antiarrhythmic effect of verapamil (Haft and Habbab 1986; Weiss et al. 1983). Calcium also influenced blood pressure by restoring it to control values when administered after treatment with verapamil (Weiss et al. 1983).

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