Milk of magnesia

Milk of Magnesia is a common name for a suspension of magnesium hydroxide (Mg(OH)₂) in water. It is widely used as an antacid and laxative, primarily for relieving indigestion, heartburn, and constipation. Known for its characteristic milky white appearance and alkaline properties, it is one of the most recognised over-the-counter gastrointestinal remedies in the world.

Chemical Composition and Properties

Milk of Magnesia is composed mainly of magnesium hydroxide, which is only slightly soluble in water. The suspended particles give the liquid its opaque, milky appearance. The typical concentration used for medicinal purposes contains about 7–8% magnesium hydroxide by weight. The chemical reaction that forms magnesium hydroxide occurs when magnesium salts, such as magnesium chloride, react with sodium hydroxide or lime water:
MgCl2+2NaOH→Mg(OH)2+2NaClMgCl_2 + 2NaOH \rightarrow Mg(OH)_2 + 2NaClMgCl2​+2NaOH→Mg(OH)2​+2NaCl
The resulting hydroxide is a weak base and reacts slowly with acids, which underpins its antacid action in neutralising excess gastric acid. The pH of a standard preparation ranges between 10 and 10.5, making it sufficiently alkaline to relieve acid discomfort without being corrosive.

Historical Background

The use of magnesium-based compounds for medicinal purposes dates back to the 19th century. Charles Henry Phillips, a British chemist, popularised the product in 1873 under the trade name Phillips’ Milk of Magnesia. Initially sold as a household remedy, it became one of the first mass-marketed pharmaceutical suspensions. Over time, it was standardised in pharmacopoeias worldwide and continues to be listed in the British Pharmacopoeia and United States Pharmacopeia.

Medical Uses and Mechanism of Action

1. As an Antacid: Milk of Magnesia neutralises stomach acid (hydrochloric acid, HCl) by forming water and magnesium chloride:
Mg(OH)2+2HCl→MgCl2+2H2OMg(OH)_2 + 2HCl \rightarrow MgCl_2 + 2H_2OMg(OH)2​+2HCl→MgCl2​+2H2​O
This neutralisation reaction provides quick relief from heartburn, acid indigestion, and sour stomach. It also helps maintain a balanced gastric environment without the excessive rebound acidity often associated with stronger alkaline compounds like sodium bicarbonate.
2. As a Laxative: In higher doses, magnesium hydroxide functions as an osmotic laxative. It draws water into the intestines, increasing bowel movement frequency and softening stool consistency. This makes it effective for short-term relief of constipation. Its action typically begins within 30 minutes to 6 hours of ingestion, depending on the dosage and individual response.

Dosage and Administration

Milk of Magnesia is commonly available in liquid form, though chewable tablets and capsules also exist. For antacid purposes, 5–15 mL is usually taken with water up to four times daily. As a laxative, doses are generally 30–60 mL taken at bedtime. Children’s doses are adjusted proportionally by weight or age, and medical advice is recommended for prolonged use.

Side Effects and Precautions

While generally safe when used appropriately, Milk of Magnesia may cause side effects in some individuals. The most frequent include:

• Diarrhoea, particularly when used in high doses.
• Abdominal cramping due to osmotic water retention in the intestines.
• Electrolyte imbalance if used excessively or over long periods.

Individuals with renal impairment should use it cautiously, as the kidneys are responsible for excreting excess magnesium. Accumulation may lead to hypermagnesaemia, a potentially serious condition characterised by weakness, hypotension, and cardiac disturbances. Pregnant or breastfeeding women are generally advised to consult healthcare providers before use.

Pharmacological Interactions

Magnesium hydroxide can interfere with the absorption of other orally administered medications. Antibiotics such as tetracyclines and fluoroquinolones form insoluble complexes with magnesium, reducing their effectiveness. Similarly, the absorption of iron supplements and certain thyroid medications can be hindered. For this reason, it is recommended that Milk of Magnesia be taken at least two hours apart from other oral drugs.

Industrial and Non-Medical Applications

Beyond medicinal use, magnesium hydroxide serves as a mild alkali in various industries. It is utilised in water treatment, fire retardants, and as a neutralising agent in chemical laboratories. Due to its non-toxic and biodegradable nature, it is often preferred in environmentally sensitive applications.

Advantages and Limitations

Advantages:

• Provides rapid relief from gastric acidity.
• Functions as a gentle, non-habit-forming laxative.
• Safer than sodium-based antacids for patients with hypertension or heart disease.

Limitations:

• Not suitable for long-term use without supervision.
• Ineffective for chronic gastrointestinal disorders requiring sustained acid control.
• May cause electrolyte imbalance with prolonged consumption.

Pharmacological Alternatives

Other magnesium-containing compounds, such as magnesium carbonate and magnesium oxide, have similar properties but differ in potency and onset of action. Aluminium-based antacids, like aluminium hydroxide, are sometimes combined with Milk of Magnesia to balance the laxative effect with mild constipating action, producing a synergistic antacid formulation.

Clinical and Therapeutic Significance

Milk of Magnesia remains one of the most trusted household remedies due to its dual functionality and safety profile. Its affordability, stability, and broad accessibility have ensured its continued inclusion in modern medical practice. It also represents an important example of how a simple inorganic compound can have multifaceted therapeutic applications rooted in basic chemistry.