VITAMIN A

Indications

• Vitamin A deficiency
• Isoniazid-induced neuropathy (prophylaxis)
• Isoniazid-induced neuropathy (treatment)
• Idiopathic sideroblastic anaemia
• Wilson's disease (treatment and prevention of penicillamine-induced neuropathy)
• Premenstrual syndrome

Dosage

Children: For children aged 1 to 11 months, 5000 units daily, to be taken with or after food. Higher doses may be used initially for treatment of severe

Adults: 20–50 mg by mouth 1–3 times a day for Vitamin A deficiency. For Isoniazid-induced neuropathy, the dosage varies: 50 mg 3 times a day for treatment, and 20 mg daily for prophylaxis.

Mechanism of action

Vitamin A acts primarily through its active metabolite, retinoic acid, which regulates gene expression by activating specific nuclear receptors known as retinoic acid receptors (RARs) and retinoid X receptors (RXRs). This pathway influences cellular growth, differentiation, and apoptosis, and is critical in the development of the visual system and immune responses.

Pharmacodynamics

Vitamin A exhibits its pharmacological effects by maintaining normal vision, supporting immune function, and promoting skin health. It is essential for the synthesis of rhodopsin, a pigment in the retina that is necessary for low-light vision. The vitamin also plays a role in the regulation of epithelial cell turnover and differentiation, influencing the health of skin and mucosal tissues.

Pharmacokinetics

Vitamin A is absorbed in the intestines, primarily in the presence of dietary fat. It is stored in the liver and released into the bloodstream bound to retinol-binding protein. The elimination half-life varies and is influenced by the amount of vitamin A stored in the body. Excessive intake can lead to toxicity, as Vitamin A is retained in the body rather than excreted.

Contra-indications

• Hyperkalaemia
• Severe liver damage
• Renal impairment (eGFR less than 45 mL/minute/1.73 m2)

Adverse effects

• Rough skin
• Dry hair
• Enlarged liver
• Increased erythrocyte sedimentation rate
• Increased serum calcium
• Increased serum alkaline phosphatase concentration

Interactions

• Isoniazid (may induce neuropathy)
• Other medications that affect liver function

Precautions

• Monitor liver function tests monthly during treatment
• Discontinue immediately if liver function tests are elevated
• Avoid high doses in pregnant women to prevent birth defects
• Monitor closely when initiating treatment with very high doses

Pregnancy

Women who are pregnant or may become pregnant are advised against taking vitamin A supplements, including tablets and fish liver oil drops, except on the advice of a doctor or antenatal clinic.

Breast-feeding

There is a theoretical risk of toxicity in infants if mothers take large doses of vitamin A.

Storage

Store in a cool, dry place, away from direct sunlight.

Formulations

• Vitamin A 150,000 units/ml drops (Arovit)
• Vitamin A (as Vitamin A palmitate) 50,000 units/ml solution for injection (Aquasol A)