Diphenoxylate, an antidiarrheal drug used to manage acute and chronic diarrhea, was developed in the mid-20th century and approved for medical use in the 1960s. Its history is marked by its effectiveness in reducing intestinal motility, but also by its potential for abuse when taken in high doses, which led to its combination with atropine to discourage misuse. Diphenoxylate, a synthetic opioid derivative, is used primarily in combination therapy for diarrhea and is available in both tablet and liquid forms. Its development included careful regulatory monitoring due to its opioid-like effects, ensuring safe prescribing and minimizing the risk of dependence.
BRAND NAMES
Lomotil – the most widely known formulation, usually combined with atropine to prevent abuse
Lonox – another combination product with similar composition
Diastat – less common, depending on regional availability.
MECHANISM OF ACTION
The mechanism of action of Diphenoxylate is primarily related to its opioid-like effects on the gastrointestinal tract. Diphenoxylate acts on μ-opioid receptors in the intestinal smooth muscle, which slows intestinal motility and increases the time for water and electrolytes to be absorbed from the stool.
PHARMACOKINETICS
Absorption
The absorption of Diphenoxylate occurs primarily through the gastrointestinal tract after oral administration. Diphenoxylate is rapidly absorbed, reaching peak plasma concentrations within 1–2 hours.
Distribution
The volume of distribution (Vd) of Diphenoxylate is relatively low to moderate, reflecting that the drug is largely confined to the gastrointestinal tract and has limited systemic distribution at therapeutic doses. Reported values for Vd are approximately 1–3 L/kg, indicating some distribution into body tissues but predominantly remaining in the gut wall and plasma.
Metabolism
The metabolism of Diphenoxylate occurs primarily in the liver. After oral administration, diphenoxylate is rapidly metabolized by hepatic enzymes to its active metabolite, difenoxin, which also contributes to the drug’s antidiarrheal effect.
Elimination
The elimination of Diphenoxylate occurs primarily via the feces and urine. After metabolism in the liver to its active metabolite difenoxin and further inactive metabolites, a significant portion of the drug is excreted in the feces, reflecting its local gastrointestinal action. The remainder is excreted renally as metabolites.
PHARMACODYNAMICS
The pharmacodynamics of Diphenoxylate are primarily related to its opioid-like effects on the gastrointestinal tract. Diphenoxylate binds to μ-opioid receptors in the intestinal smooth muscle, leading to decreased peristalsis and slower transit of intestinal contents.
ADMINISTRATION
The typical regimen involves taking the medication after each loose stool, with the dosage adjusted according to patient age and response. For adults, the usual starting dose is 2 tablets (2.5 mg diphenoxylate each) or 5 mL of liquid every 4 hours as needed, not exceeding the maximum daily limit.
DOSAGE AND STRENGTH
The dosage and strength of Diphenoxylate vary according to age and formulation. For adults and children over 13 years, the usual oral dose is 2 tablets (2.5 mg diphenoxylate each) or 5 mL of liquid taken after each loose stool, with a maximum daily limit of 8 tablets or 20 mL. Children between 2 and 12 years receive lower, weight-appropriate doses, typically half of the adult dose.
DRUG INTERACTIONS
he drug interactions of Diphenoxylate are primarily related to its opioid-like effects and potential for CNS depression. Concomitant use with other central nervous system depressants, such as alcohol, sedatives, or opioids, can increase drowsiness, dizziness, and the risk of respiratory depression.
FOOD INTERACTIONS
Diphenoxylate can be taken with or without meals, though taking it with food may help reduce mild gastrointestinal discomfort in some patients.
CONTRAINDICATIONS
The contraindications of Diphenoxylate include conditions where its use may be unsafe or exacerbate underlying health problems. It is contraindicated in patients with known hypersensitivity to diphenoxylate or other opioids. It should not be used in children under 2 years of age due to the risk of severe respiratory depression.
SIDE EFFECTS
Drowsiness or sedation
Dizziness
Dry mouth
Blurred vision
Nausea and vomiting
Constipation
Abdominal discomfort
OVER DOSAGE
Overdosage of Diphenoxylate can lead to serious opioid-like and anticholinergic effects, particularly when taken in doses exceeding the recommended limits. Early symptoms may include extreme drowsiness, dizziness, dry mouth, blurred vision, nausea, vomiting, and constipation. Severe overdose can result in respiratory depression, profound sedation, confusion, seizures, coma, and in rare cases, death.
TOXICITY
Toxicity of Diphenoxylate results mainly from excessive opioid-like and anticholinergic effects, usually due to overdose or prolonged high-dose use. Clinical manifestations include severe sedation, dizziness, confusion, respiratory depression, blurred vision, dry mouth, constipation, and in extreme cases, coma or death. Cardiovascular effects such as tachycardia or hypotension may also occur.
