Cathartics are substances which promote bowl movement and help relieve constipation. They work by increasing the movement of the intestines or by softening the stool, making it easier to pass. They often used for short term relief of constipation or to prepare for certain medical procedures. In normal habits, peristalsis lead to defecation. Peristaltic movement normally occur about three to four times a day. By ignoring the urge to defecate voluntarily or for psychological reason leads to constipation. Constipation may also be caused by many factors like weakness of intestine, intestinal spasm, injury or use of certain drugs, etc. In constipation, faecal matters become dry and hard. Use of cathartics gives relief in constipation.

Mechanism of cathartics

Cathartics generally act by four different mechanisms as follows,

Stimulant: In this type the drugs or chemical act by local irritation on intestinal tract. This irritation brings out stimulation of peristalsis activity. Hence, they are called as stimulants. For example, senna, rhubarb, cascara, castor oil, aloe, bisacodyl, etc.

Bulk purgatives: These are the agents which increases the bulk of intestinal contents. These are cellulose or non-digestible type of materials which swell considerably when wet. For example, methyl cellulose, sodium carboxymethyl cellulose, gum, isapgol, etc.

Lubricants: In constipation, contents of intestine become hard due to absorption of water by body. Hence, this results in difficulty to clear the bowels. Lubricants brings out smooth clearance of the faecal matter. For example, liquid paraffin, glycerine, mineral oils, etc.

Saline cathartics: the saline cathartics act by increasing the osmotic load of intestine, by absorbing large quantity of water stimulating peristalsis. These are water soluble inorganic chemicals and they are administered with plenty of water. This helps in restricting excessive loss of body fluids and reduces nausea and vomiting. For example, poorly absorbable cations like calcium, magnesium and anions like phosphate, sodium tartrate.

Magnesium sulphate (Epsom salt)

Molecular formula: MgSO4.7H2O

Molecular weight: 246.47


It is obtained by the action of dilute sulphuric acid on magnesium carbonate or magnesium oxide.

MgCO3 + H2SO4 — > MgSO4 + H2O + CO2


  • Colourless crystals
  • Saline and bitter in taste
  • Effloresces in dry air
  • Freely soluble in water
  • Sparingly soluble in alcohol

Uses: Laxative and also used as antidote in heavy metal poisoning

Dose: Usual dose is 10-15gm


It is analysed by complexometric titration using disodium edetate as titrant and mordant black 11 as indicator. It contains not less than 99% and not more than 100% of MgSO4 calculated with reference to the ignited substance.

Sodium orthophosphate

Molecular Formula: NaH₂PO₄

Molecular Weight: 119.98 g/mol


It can be prepared by combining sodium carbonate (soda ash) with phosphoric acid. Sodium carbonate reacts with phosphoric acid to form sodium orthophosphate (disodium orthophosphate).

H3PO4 + Na2CO3 — > Na2HPO4 + H2O + CO2

Physical Properties

Appearance: Off-white powder.

Odor: Odorless.

Density: Approximately 1.62 g/cm³.

Melting Point: 1,583 °C.

Solubility: Highly soluble in water


  • Cathartic: Sodium orthophosphate acts as a cathartic, promoting bowel movements and relieving constipation.
  • Medical Applications: It is used before colonoscopies to cleanse the intestines.
  • Buffer: Sodium phosphate buffers are widely used in pharmaceutical formulations.
  • Antihypercalcemia: Used to control high blood calcium levels.

Assay (for monosodium orthophosphate)

  • Dissolve about 2.5 g of monosodium dihydrogen orthophosphate in 10 mL of cold water.
  • Add 20 mL of a cold, saturated solution of sodium chloride.
  • Add phenolphthalein indicator.
  • Titrate the solution with 1 N sodium hydroxide while keeping the temperature between 10°C and 15°C during the entire titration.
  • Perform a blank determination and make any necessary corrections.
  • Each mL of 1 N sodium hydroxide is equivalent to 120.0 mg of NaH₂PO₄.


Molecular Formula: Al₂Si₂O₅(OH)₄ (in mineralogy) or Al₂O₃·2SiO₂·2H₂O (in oxide notation).

Molecular Weight: Approximately 222.13 g/mol.


  • Mining and Excavation: The raw kaolin rock is mined and excavated from natural deposits.
  • Washing and Separation: The impurities in the raw kaolin are washed out with water. The mixture is then powdered.
  • Elutriation and Separation: The kaolin rock is elutriated with water. Large-sized particles are separated from the mixture.
  • Settling and Drying: The turbid liquid is allowed to settle. Heavy kaolin particles containing large sizes are separated. The separated kaolin is then dried.


  • Color: Naturally bright white (may have yellow or rust-colored stains due to iron oxide).
  • Hardness: Low (similar to gypsum; can be scratched with a fingernail).
  • Texture: Soft and earthy.
  • Odor: Odorless.
  • Density: Approximately 1.62 g/cm³.
  • Kaolin clay is highly adsorbent.
  • Kaolin is chemically inert and has a neutral pH level.
  • Kaolin’s crystal structure prevents water infiltration, giving it non-swelling properties.


  • Diarrhea Control: Kaolin has traditionally been used internally to control diarrhea. It acts as an adsorbent to bind gastrointestinal toxins and regulate bowel movements.
  • Topical Applications: Emollient and Drying Agent: Kaolin is used topically as an emollient and drying agent.
  • Skin Conditions: It helps dry oozing and weeping lesions caused by poison ivy, poison oak, and poison sumac.
  • Anorectal Itching and Diaper Rash: Kaolin serves as a protectant for temporary relief from anorectal itching and diaper rash.
  • Radiation- and Chemotherapy-Induced Mucositis: A mixture of kaolin/pectin and diphenhydramine is used to manage mucositis caused by radiation or chemotherapy. The mixture is held in the mouth for 3 minutes.


The assay method for kaolin involves the Kaolin Clotting Time (KCT) test. Here are the details.


  • The KCT is essentially an Activated Partial Thromboplastin Time (APTT) test but without any added phospholipid.
  • It relies on residual cell membrane fragments and plasma lipids to provide a phospholipid surface for coagulation reactions.
  • Kaolin suspension is turbid, making it difficult to use optical density measurements to determine the endpoint, so the test is usually performed manually.


Molecular Formula: Al₂O₃·4(SiO₂)·H₂O

Molecular Weight: Approximately 360.31 g/mol


  • Mining and Excavation: The raw bentonite rock is mined and excavated from natural deposits.
  • Washing and Separation: Impurities in the raw bentonite are washed out with water. The mixture is then powdered.
  • Elutriation and Separation: The bentonite rock is elutriated with water. Large-sized particles are separated from the mixture.
  • Settling and Drying: The turbid liquid is allowed to settle. Heavy bentonite particles containing large sizes are separated. The separated bentonite is then dried.


  • Color: Usually light gray or beige, but can also be green or reddish.
  • Texture: Soft and fine; easily crumbled.
  • Density: Generally, between 2.0 and 2.7 g/cm³.
  • Bentonite is chemically inert and has a neutral pH level.
  • Bentonite is highly adsorbent.


  • Constipation: Bentonite clay can improve constipation by promoting regular bowel movements.
  • Irritable Bowel Syndrome (IBS): It may help manage IBS symptoms.
  • Bloating and Gas: Bentonite clay may alleviate bloating and gas.
  • Bentonite clay is used for detoxifying the body by binding to toxins and heavy metals.


The assay method for bentonite involves the Kaolin Clotting Time (KCT) test.


Cathartics (also known as laxatives or purgatives) are substances used to treat and prevent constipation by loosening stools and increasing bowel movements. They are employed to ease defecation in patients with painful haemorrhoids, prevent excessive straining in patients with hernias, relieve acute constipation, and prepare for certain roentgenographic studies. Cathartics can be classified into four types: laxatives (milder action), purgatives (moderate action), cathartics (stronger action), and bulk-forming agents. Examples of cathartics include magnesium sulphate, sodium orthophosphate, kaolin, and bentonite. These agents work by increasing water content in faeces, stimulating peristalsis, and promoting bowel movement.

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