Liquid paraffin

Indications

• Insecticide for agricultural use
• Research tool in toxicology

Dosage

Children: Refer to the BNF for Children for specific dosing and administration guidelines.

Adults: Refer to the BNF for specific dosing and administration guidelines.

Mechanism of action

Methyl parathion acts primarily by inhibiting the enzyme acetylcholinesterase, which is essential for the breakdown of the neurotransmitter acetylcholine. Its active metabolite, methyl paraoxon, is a potent inhibitor of both acetylcholinesterase and butyrylcholinesterase. The inhibition of these enzymes results in the accumulation of acetylcholine at synapses, leading to overstimulation of cholinergic receptors and resultant toxic effects.

Pharmacodynamics

The pharmacodynamics of methyl parathion involve its action as a noncompetitive inhibitor of acetylcholinesterase, causing prolonged effects of acetylcholine due to its inability to be hydrolyzed. The resultant cholinergic toxicity can lead to symptoms such as muscle twitching, respiratory distress, and potentially fatal outcomes if not treated promptly. The extent of inhibition is dose-dependent, with significant toxicity occurring after substantial enzyme inhibition.

Pharmacokinetics

Methyl parathion is absorbed through the gastrointestinal tract and can also be absorbed through the skin and respiratory tract. It is metabolized in the liver to form methyl paraoxon, which is responsible for the majority of its toxic effects. The distribution of methyl parathion in body tissues is influenced by its lipophilicity, and it is primarily excreted as metabolites in the urine. The elimination half-life and specific pharmacokinetic parameters can vary based on individual metabolism and exposure levels.

Pregnancy

There are no adequate and well-controlled studies in pregnant women. Use only if the potential benefit justifies the potential risk to the fetus.

Breast-feeding

It is not known whether this drug is excreted in human milk. Caution is advised when administering to nursing women.

Storage

Store in a cool, dry place away from direct sunlight. Keep out of reach of children.

Formulations

• Liquid formulation

Indications

• Constipation
• Dry skin
• Skin irritation

Dosage

Children: Refer to specific guidelines and prescribing information for paediatric dosing.

Adults: Refer to specific guidelines and prescribing information for adult dosing.

Mechanism of action

Paraffin acts as a lubricating agent in the gastrointestinal tract. It coats the stool and the intestinal walls, which helps to ease the passage of feces by reducing friction. This action promotes bowel movements and alleviates constipation. When used topically, it forms a barrier on the skin, which helps to retain moisture and protect against irritants.

Pharmacodynamics

Paraffin has a low viscosity and surface tension, which allows it to spread easily over surfaces. Its lubricating properties facilitate the movement of stool through the intestines, while its emollient properties help in maintaining skin hydration and barrier function. The onset of action for oral administration typically occurs within 6 to 8 hours, making it effective in treating occasional constipation.

Pharmacokinetics

Paraffin is not absorbed systemically when ingested; it remains in the gastrointestinal tract and is excreted unchanged in the feces. After oral administration, it acts locally in the intestines without significant systemic effects. When used topically, it remains on the skin surface and does not penetrate deeply, providing a protective layer without altering systemic pharmacokinetics.

Adverse effects

• Abdominal cramps
• Diarrhea
• Nausea
• Vomiting
• Lipid pneumonia (when aspirated)
• Electrolyte imbalances

Precautions

• Use with caution in patients with gastrointestinal obstruction
• Avoid in patients with a history of aspiration
• Monitor for signs of dehydration with prolonged use

Pregnancy

Use only if clearly needed. Consult a healthcare provider for advice.

Breast-feeding

Paraffin can be excreted in breast milk, use with caution.

Storage

Store at room temperature away from moisture and heat.

Formulations

• Liquid paraffin
• Soft paraffin (for topical use)

Mechanism of action

Acute poisoning ... is related to ... inhibiting action on enzyme acetylcholinesterase. Toxic manifestations generally occur only after more than 50% of plasma cholinesterase is inhibited. ... Methyl parathion ... depend on oxidative activation by replacement of thiono-sulfur with oxygen for ... toxicity. Methyl parathion has only a slight inhibitory action on acetylcholinesterase and butyrylcholinesterase, but its active metabolite, methyl paraoxon, is a potent inhibitor of both these enzymes. A study was conducted examining the inhibition of (Ca2+ and Mg2+)-ATPase by parathion (56382) and methyl parathion. Enzyme activity was assessed spectrophotometrically in pig erythrocyte membranes containing calcium2+ (Ca2+) and magnesium2+ and in solubilized membrane preparations incubated with the test agents. The enzyme response to ATP was biphasic. Equations expressing the kinetics of the substrate curves described two classes of the ATP binding active site, one with high affinity and low maximum rate and one with low affinity and high maximum rate. High affinity active sites were stimulated by low ATP concentrations (20 uM), whereas low affinity active sites were stimulated by high ATP levels (2 mM). Parathion and methylparathion dose dependently inhibited enzyme activity; parathion had a greater inhibitory effect than methylparathion. Lineweaver-Burke and Dixon plots indicated noncompetitive inhibition. Parathion and methylparathion induced enzyme inhibition occurred over a range of free calcium ion concentrations (0.5 to 5 mM); the inhibition was significantly greater at lower Ca2+ concentrations (1 to 100 uM) than at higher concentrations. The authors conclude that parathion and methylparathion inhibit ATPase activity by binding to a site on the enzyme rather than through an interaction with associated lipids. For more Mechanism of Action (Complete) data for METHYL PARATHION (6 total), please visit the HSDB record page.