Magnesium Hydroxide (Milk of Magnesia): Uses, Dosage, Side Effects

Overview

Magnesium hydroxide is an over-the-counter antacid and laxative medication used to neutralize stomach acid and relieve heartburn, indigestion, and upset stomach. It is the active ingredient in Phillips’ Milk of Magnesia, one of the oldest and most recognized antacid brands.

Magnesium hydroxide works quickly to neutralize existing stomach acid and also has a laxative effect at higher doses, making it useful for both occasional heartburn relief and constipation treatment.

What Is Magnesium Hydroxide?

Type: Antacid and saline laxative

Primary uses:

Heartburn relief
Acid indigestion
Sour stomach
Upset stomach
Constipation (at laxative doses)

Brand name: Phillips’ Milk of Magnesia (most common)

Availability: Over-the-counter in liquid and chewable tablet forms

How it works: Directly neutralizes stomach acid and draws water into intestines

How Magnesium Hydroxide Works

As an Antacid

Acid neutralization mechanism:

According to antacid pharmacology literature, when magnesium hydroxide enters the stomach:

Reacts directly with hydrochloric acid (stomach acid)
Forms magnesium chloride and water
Chemical reaction: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
Neutralizes acid rapidly (within minutes)
Raises stomach pH from acidic (1-3) to less acidic (3-5)

Buffering effect:

Provides immediate relief of acid-related symptoms
Does not prevent acid production (unlike PPIs)
Effect lasts 30 minutes to 3 hours depending on stomach contents
Works only on existing acid, not future acid production

As a Laxative

Osmotic laxative mechanism:

According to FDA-approved laxative monograph, at higher doses:

Magnesium draws water into intestinal lumen (osmotic effect)
Increases intestinal fluid volume
Stimulates bowel movements
Softens stool
Usually produces bowel movement within 30 minutes to 6 hours

Dual action:

Antacid doses: 400-800 mg (low dose, minimal laxative effect)
Laxative doses: 2,400-4,800 mg (high dose, strong laxative effect)

Common Uses

Heartburn Relief

Primary antacid use:

Occasional heartburn after meals
Acid indigestion from overeating
Upset stomach from acidic foods
Sour stomach

Best for:

Immediate symptom relief
Occasional symptoms (not frequent or chronic)
Symptoms that occur unpredictably
Quick relief needed

FDA-approved indication: According to FDA OTC antacid monograph, magnesium hydroxide is approved for relief of heartburn, acid indigestion, sour stomach, and upset stomach associated with these symptoms.

Acid Indigestion

Digestive discomfort:

Post-meal bloating with acid
Burning sensation in upper abdomen
Discomfort from spicy or acidic foods
General stomach upset

Constipation Relief

As a laxative:

Occasional constipation
Gentle bowel stimulation
Short-term use only
Works faster than many other laxatives

Laxative effectiveness: According to medical literature on osmotic laxatives, magnesium hydroxide is effective for occasional constipation, typically producing bowel movements within 30 minutes to 6 hours.

Who Benefits Most

Magnesium hydroxide is most helpful for:

People with occasional heartburn needing immediate relief
Those who prefer liquid antacids over tablets
Individuals needing both antacid and mild laxative effects
People who experience constipation from calcium-based antacids
Anyone wanting fast-acting heartburn relief
Those who cannot or prefer not to swallow tablets

Effectiveness

According to antacid clinical studies and FDA monographs, magnesium hydroxide is highly effective:

Strong evidence for:

Rapid heartburn relief (works within 5-10 minutes)
Acid neutralization (high acid-neutralizing capacity)
Symptom relief for occasional acid-related discomfort
Short-term constipation relief

Advantages:

Very fast onset (5-10 minutes)
Strong acid-neutralizing capacity
Liquid form works faster than tablets
Dual antacid/laxative effect can be beneficial
Long history of safe use
Inexpensive

Limitations:

Short duration (30 minutes to 3 hours)
Laxative effect may be unwanted for some users
Not for frequent or chronic symptoms
Does not heal acid-damaged tissue
Requires multiple daily doses for ongoing relief
Can cause diarrhea

How It Differs from Calcium Carbonate

Magnesium Hydroxide

Characteristics:

Fast onset (5-10 minutes)
Strong acid neutralization
Laxative side effect (can cause diarrhea)
Liquid form most common
May be absorbed in small amounts
Caution with kidney disease

Calcium Carbonate

Characteristics:

Fast onset (similar timing)
Very strong acid neutralization
Constipating side effect
Tablet form most common
Provides calcium supplementation
Generally safer for kidney disease

For detailed comparison, see: Calcium Carbonate vs Magnesium Hydroxide

How It Differs from Other Acid Reducers

Magnesium Hydroxide vs H2 Blockers (Famotidine)

Magnesium hydroxide:

Neutralizes existing acid
Works within 5-10 minutes
Lasts 30 minutes to 3 hours
For occasional symptoms
No drug interactions

H2 blockers:

Reduce acid production
Work in 30-60 minutes
Last 6-12 hours
For frequent symptoms
Possible drug interactions

Magnesium Hydroxide vs PPIs (Omeprazole)

Magnesium hydroxide:

Immediate acid neutralization
Works in minutes
Short duration
For occasional use
No long-term risks

PPIs:

Block acid production
Take 1-4 days for full effect
Last 24+ hours
For frequent/chronic symptoms
Long-term use considerations

Onset and Duration

Time to work: According to antacid pharmacology data:

Begins working within 5-10 minutes
Liquid form may work faster than chewable tablets
Peak effect at 10-20 minutes
Immediate symptom relief in most cases

Duration:

Effects last 30 minutes to 3 hours
Duration depends on stomach contents
Shorter duration when taken on empty stomach
Longer duration when taken with food
May need redosing every few hours for sustained relief

Forms Available

Liquid Suspension

Most common form:

Original “Milk of Magnesia” liquid
Typical concentration: 400 mg per 5 mL (teaspoon)
1,200 mg per tablespoon (antacid dose)
Works very quickly
Can be mixed with water or milk
Pleasant mint or cherry flavor options

Chewable Tablets

Tablet form:

311 mg or 400 mg typical strengths
Must be chewed thoroughly
May work slightly slower than liquid
More portable
No measuring required
Good for on-the-go use

Concentrated Liquid

Higher concentration:

More magnesium hydroxide per dose
Smaller volume to swallow
Same effectiveness
Convenient for regular users

Liquid is traditionally preferred for fastest action and ease of use.

Safety Profile

According to FDA OTC antacid monograph and medical literature, magnesium hydroxide is generally safe for occasional use:

Well-tolerated for short-term use:

Decades of safe OTC availability
Good safety record when used as directed
Appropriate for most adults
Safe for occasional use during pregnancy

Common side effect:

Diarrhea or loose stools (laxative effect)
Usually mild with antacid doses
More common with higher doses

Important cautions:

Kidney disease (magnesium accumulation risk)
Chronic use (electrolyte imbalances)
High-dose or long-term use

Generally safe for:

Most adults
Occasional use (not daily for extended periods)
Pregnancy (short-term, appropriate doses)
Older adults without kidney disease

Drug Interactions

According to FDA guidance and drug interaction databases:

Can reduce absorption of:

Tetracycline antibiotics
Fluoroquinolone antibiotics (ciprofloxacin, levofloxacin)
Iron supplements
Some thyroid medications
Certain heart medications (digoxin)
Bisphosphonates (osteoporosis medications)

Recommendation:

Separate magnesium hydroxide from other medications by 2 hours
Take medications first, then antacid 2 hours later
Or take medications 2 hours after antacid
Consult pharmacist about specific medications

Medications to discuss with healthcare provider:

Antibiotics
Thyroid medications
Iron supplements
Osteoporosis medications
Heart medications

Special Considerations

Pregnancy

Pregnancy Category: According to FDA guidance, magnesium hydroxide is generally considered safe during pregnancy:

Used for occasional heartburn in pregnancy
Short-term use considered low risk
Avoid excessive or prolonged use
Consult obstetrician if using frequently
Heartburn very common in pregnancy
Alternative to medications absorbed systemically

Breastfeeding

Nursing mothers:

Minimal absorption into bloodstream
Considered compatible with breastfeeding
Short-term, occasional use is safe
Consult healthcare provider for frequent use

Children

Pediatric use: According to FDA OTC labeling:

Not recommended under age 12 without medical supervision
Children’s dosing requires healthcare provider guidance
Safety and appropriate dosing less established in young children

Older Adults

Seniors:

Safe for occasional use
Check kidney function if using regularly
Higher risk of magnesium accumulation if kidney disease present
May be preferred over calcium carbonate if constipation is a concern
Monitor for diarrhea or electrolyte issues

Kidney Disease

Important warning: According to medical literature and FDA guidance:

Magnesium can accumulate in kidney disease
Avoid regular use with impaired kidney function
Occasional use may be acceptable with mild kidney disease
Consult healthcare provider before use
Calcium carbonate may be safer alternative

Signs of magnesium accumulation:

Nausea
Muscle weakness
Confusion
Low blood pressure
Irregular heartbeat

When Not to Use Magnesium Hydroxide

Do not use if:

Severe kidney disease or kidney failure
Allergy to magnesium hydroxide
Currently have diarrhea
Abdominal pain of unknown cause
Symptoms of appendicitis
Taking tetracycline antibiotics (without 2-hour separation)

Use caution if:

Mild to moderate kidney disease
On restricted magnesium diet
Taking multiple medications
Elderly with borderline kidney function

Antacid vs Laxative Dosing

Antacid Dose

For heartburn/acid indigestion:

400-1,200 mg (1-3 teaspoons of liquid)
Minimal laxative effect at this dose
Can repeat as needed
Maximum based on product label

Laxative Dose

For constipation:

2,400-4,800 mg (2-4 tablespoons of liquid)
Strong laxative effect expected
Usually works within 30 minutes to 6 hours
Do not exceed recommended dose

Important: The same product is used for both purposes but at different doses.

Long-Term Use Considerations

Magnesium hydroxide is intended for occasional use only.

According to FDA OTC monograph and medical guidance:

Risks of chronic daily use:

Diarrhea and dehydration
Electrolyte imbalances
Magnesium accumulation (especially with kidney disease)
Phosphate depletion
Dependency for bowel movements (when used as laxative)

If frequent use needed:

Symptoms occurring 2+ times per week require medical evaluation
May indicate GERD or other condition needing different treatment
H2 blockers or PPIs more appropriate for frequent symptoms
See healthcare provider for proper diagnosis

Combination Products

Magnesium hydroxide is often combined with other antacids:

Magnesium hydroxide + Aluminum hydroxide:

Balances side effects (magnesium’s laxative effect counters aluminum’s constipating effect)
Common combination (Maalox, Mylanta)
May reduce diarrhea risk
Broader symptom relief

Magnesium hydroxide + Simethicone:

Adds anti-gas effect
Helpful for bloating with heartburn
Simethicone breaks up gas bubbles

Tips for Best Results

To maximize effectiveness:

Shake liquid well before each use
Measure accurately using provided cup or spoon
Take as needed when symptoms occur
Timing: Can take with or without food
Separate from other medications by 2 hours
Chew tablets thoroughly if using chewable form
Follow with water to help distribution
Don’t exceed maximum daily dose on label

Comparison with Other Antacids

Acid-neutralizing capacity (per dose):

Magnesium hydroxide: High
Calcium carbonate: Very high
Aluminum hydroxide: Moderate
Sodium bicarbonate: Moderate to high

Speed of onset:

Magnesium hydroxide: 5-10 minutes
Calcium carbonate: 5 minutes
Aluminum hydroxide: 15-30 minutes
Sodium bicarbonate: Immediate (1-2 minutes)