Action

Increases osmotic gradient in small intestine, which draws water into intestines and causes distention. These effects stimulate peristalsis and bowel evacuation. In antacid action, reacts with hydrochloric acid in stomach to form water and increase gastric pH. In anticonvulsant action, depresses CNS and blocks transmission of peripheral neuromuscular impulses.

Availability

magnesium chloride

Injection: 20%

magnesium citrate

Oral solution: 240-ml, 296-ml, and 300-ml bottles

magnesium gluconate

Liquid: 1,000 mg/5 ml

Tablets: 500 mg

magnesium hydroxide

Liquid: 400 mg/5 ml

Liquid concentrate: 800 mg/5 ml

Tablets (chewable): 300 mg

magnesium oxide

Capsules: 140 mg

Tablets: 250 mg, 400 mg, 420 mg, 500 mg

magnesium sulfate

Granules (for oral use): 120 g, 4 lb

Injection: 10%, 12.5%, 25%, 50%

Indications and dosages

➣ Mild magnesium deficiency

Adults: 1 g (2 ml of 50% sulfate solution) I.M. q 6 hours for four doses

➣ Severe hypomagnesemia

Adults: 250 mg (2 mEq)/kg (sulfate) I.M. within 4-hour period, or 5 g (approximately 40 mEq) in 1 liter 5% dextrose injection or 0.9% sodium chloride solution by I.V. infusion over 3 hours

➣ Hypomagnesemia treatment

Adults and children: Dosage individualized based on severity of deficiency; may give citrate, gluconate, hydroxide, oxide, or sulfate.

➣ Hypomagnesemia prophylaxis

Adults and children: Dosage based on normal recommended daily magnesium intake; may give citrate, gluconate, hydroxide, oxide, or sulfate.

➣ Supplemental magnesium in total parenteral nutrition (TPN)

Adults: 8 to 24 mEq/day (sulfate) by I.V. infusion, added to TPN solution

➣ Constipation

Adults and children ages 12 and older: 15 g (sulfate granules) in 240 ml water; or 30 to 60 ml/day P.O. (hydroxide) given with water; or a single dose of 10 to 30 ml P.O. (hydroxide concentrate); or one bottle of oral solution (citrate), as directed

Children ages 6 to 11: 5 to 10 g (sulfate granules) in 120 ml water; or a single dose of 2.5 to 5 ml P.O. (sulfate) in a half-glass of water; or 15 to 30 ml P.O. daily (hydroxide) given with water; or a single dose of 7.5 to 15 ml P.O. (hydroxide concentrate); or three to four tablets (hydroxide); or 50 to 100 ml, as directed, of oral solution (citrate)

Children ages 2 to 5: Single dose of 5 to 15 ml P.O. (hydroxide); or 2.5 to 7.5 ml P.O. daily (hydroxide concentrate); or one to two tablets (hydroxide); or 4 to 12 ml oral solution (citrate), as directed

➣ Indigestion

Adults and children ages 12 and older: 5 to 15 ml P.O. (hydroxide liquid) up to q.i.d. with water; or 2.5 to 7.5 ml P.O. (hydroxide liquid concentrate) up to q.i.d. with water; or 622 to 1,244 mg P.O. (hydroxide tablets) up to q.i.d.; or 400 to 800 mg P.O. (oxide tablets) daily

➣ To prevent and control seizures in preeclampsia or eclampsia

Adults: 4 to 5 g 50% sulfate solution I.M. q 4 hours, as necessary; or 4 g 10% to 20% sulfate solution I.V., not to exceed 1.5 ml/minute of 10% solution; or 4 to 5 g I.V. infusion in 250 ml of 5% dextrose or sodium chloride solution, not to exceed 3 ml/minute

➣ Acute nephritis to control hypertension, encephalopathy, and seizures in children

Children: 100 mg/kg 50% sulfate solution I.M. q 4 to 6 hours as needed; or 20 to 40 mg/kg 20% solution I.M., repeated as necessary

Off-label uses

• Bronchodilation in some asthmatic patients

• Post-myocardial infarction hypomagnesemia

Contraindications

• Hypermagnesemia

• Heart block

• Myocardial damage

• Active labor or within 2 hours of delivery

Precautions

Use cautiously in:

• renal insufficiency, abdominal pain, nausea and vomiting, rectal bleeding, anuria, hypocalcemia

• pregnant patients.

Administration

See Be aware that magnesium sulfate injection is a high-alert drug.

• Know that I.V. use is reserved for life-threatening seizures.

• When giving magnesium sulfate I.V., don't exceed concentration of 20% or infusion rate of 150 mg/minute, except in seizures caused by severe eclampsia. Too-rapid I.V. infusion may cause hypotension and asystole.

• When giving magnesium sulfate I.M. to adults, use concentration of 25% to 50%; when giving to infants and children, don't exceed 20%.

Adverse reactions

CNS (with I.V. use): confusion, decreased reflexes, dizziness, syncope, sedation, hypothermia, paralysis

CV (with I.V. use): hypotension, arrhythmias, circulatory collapse

GI: nausea, vomiting, cramps, flatulence, anorexia

Metabolic: hypermagnesemia, hypocalcemia

Musculoskeletal (with I.V. use): muscle weakness, flaccidity

Respiratory: respiratory paralysis Skin: diaphoresis

Other: allergic reaction, injection site reaction, laxative dependence (with repeated or prolonged use)

Interactions

Drug-drug. Aminoquinolones, nitrofurantoin, penicillamine, tetracyclines: decreased absorption of these drugs (with oral magnesium)

CNS depressants: additive effects

Digoxin: heart block, conduction changes (with I.V. use)

Enteric-coated drugs: faster dissolution of these drugs

Neuromuscular blockers: increased effects of these drugs (with I.V. use)

Drug-diagnostic tests. Calcium, magnesium: increased levels (with I.V. use)

Patient monitoring

See When giving prolonged or repeated I.V. infusions, assess patellar reflex and monitor for respiratory rate of 16 breaths/minute or more.

See With I.V. use, monitor blood magnesium level (desired level is 3 to 6 mg/dl or 2.5 to 5 mEq/L). Check for signs and symptoms of magnesium toxicity (hypotension, nausea, vomiting, ECG changes, muscle weakness, mental or respiratory depression, coma). Keep injectable calcium on hand to counteract magnesium toxicity.

• Monitor urine output, which should measure 100 ml or more every 4 hours.

See If I.V. magnesium was given before delivery, assess neonate for signs and symptoms of magnesium toxicity, such as neuromuscular or respiratory depression.

• Monitor electrolyte levels and liver function tests.

Patient teaching

See Teach patient about adverse reactions. Instruct him to report symptoms that occur during I.V. administration.

• Advise patient to consult prescriber before using magnesium if he's taking other drugs. Magnesium may delay or enhance absorption of other drugs.

• Inform patient that repeated or prolonged use of magnesium citrate, hydroxide, or sulfate may cause laxative dependence. Inform him that healthy diet and exercise can reduce need for laxatives.

• Tell pregnant female to make sure prescriber knows she is pregnant before taking drug.

• As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs and tests mentioned above.