Prazosin

BRAND NAMES

• Minipress: The most widely recognized brand of prazosin in the United States and worldwide, produced by Pfizer.

• Minipress XL: An extended-release formulation of prazosin for longer-lasting effects.

• Minizide: A combination medication that contains prazosin along with the diuretic polythiazide.

MECHANISM OF ACTION

Pyridoxine (Vitamin B6) functions as a coenzyme in its active form, pyridoxal 5′-phosphate (PLP). It participates in amino acid, carbohydrate, and lipid metabolism, including transamination, decarboxylation, and racemization reactions. It is essential for the synthesis of neurotransmitters like serotonin, dopamine, GABA, and norepinephrine. Pyridoxine also plays a role in hemoglobin production and modulation of homocysteine levels. Adequate vitamin B6 ensures proper nervous system function and cellular metabolism.

PHARMACOKINETICS

Absorption: Pyridoxine is well absorbed in the jejunum from the gastrointestinal tract, except in malabsorption conditions.

Distribution: The active metabolite, pyridoxal 5′-phosphate, is highly protein-bound, mainly to albumin, and constitutes most circulating vitamin B6.

Metabolism: Pyridoxine is metabolized primarily in the liver, forming active and inactive metabolites; the main inactive metabolite is 4-pyridoxic acid.

Excretion: Excess pyridoxine and its metabolites are excreted in urine, preventing accumulation under normal intake.

PHARMACODYNAMICS

Vitamin B6 acts as a coenzyme in numerous enzymatic reactions affecting amino acid, neurotransmitter, and hemoglobin synthesis. It contributes to energy production and proper nerve function. The biologically active form, PLP, is essential for neurotransmission and metabolic balance. Pyridoxine helps regulate homocysteine levels, reducing cardiovascular risk. Deficiency can lead to neurological, hematological, and immune system abnormalities.

ADMINISTRATION

Pyridoxine can be administered orally, intramuscularly (IM), intravenously (IV), or subcutaneously (SC). The route depends on the severity of deficiency and patient condition. Oral administration is preferred for routine supplementation. Injectable forms are used when oral intake is not possible or rapid correction is required. Dosage and frequency are adjusted based on clinical response and underlying conditions.

DOSAGE AND STRENGTH

For adults, oral doses typically range from 10–50 mg daily for deficiency prevention or treatment. Children’s doses are weight-based and adjusted according to clinical needs. For isoniazid-induced neuropathy, 10–25 mg daily is recommended. Pyridoxine is available in 10 mg, 25 mg, and 50 mg tablets as well as injectable solutions. Higher doses may be prescribed under supervision for specific metabolic or neuropathic conditions.

DRUG INTERACTIONS

Pyridoxine may interact with isoniazid, hydralazine, penicillamine, cycloserine, and levodopa. Isoniazid and similar drugs can cause vitamin B6 deficiency, requiring supplementation. High doses of pyridoxine may reduce levodopa’s effectiveness unless combined with a decarboxylase inhibitor. It can also affect the toxicity or efficacy of theophylline and other medications. Careful monitoring is advised when pyridoxine is co-administered with interacting drugs.

FOOD INTERACTIONS

Pyridoxine is generally well absorbed from a balanced diet. Excessive consumption of raw egg whites may reduce absorption due to the protein avidin. Alcohol intake can interfere with vitamin B6 metabolism and increase deficiency risk.

CONTRAINDICATIONS

Pyridoxine is contraindicated in patients with hypersensitivity to the vitamin or its formulations. Caution is required in renal or hepatic impairment, as metabolism and excretion may be affected. High doses should be avoided in individuals with peripheral neuropathy of unknown cause. Overuse can lead to toxicity and sensory nerve damage. It should be used under medical supervision in pregnancy, lactation, or chronic illness.

SIDE EFFECTS

• Dizziness, especially when standing up (orthostatic hypotension)

• Headache

• Fatigue or drowsiness

• Nausea or upset stomach

• Palpitations or rapid heartbeat

OVER DOSE

An overdose of prazosin primarily results in excessive hypotension due to profound vasodilation. Symptoms may include severe dizziness, fainting, weakness, blurred vision, and palpitations. Marked postural hypotension and syncope are common, especially when standing. In severe cases, shock or collapse may occur. Management is supportive and includes placing the patient in a supine position, monitoring vital signs, and providing intravenous fluids if necessary.

TOXICITY

Prazosin toxicity is mainly related to its pharmacological action of alpha‑1 blockade, leading to dangerous drops in blood pressure. Central nervous system effects such as drowsiness, confusion, and lethargy may be observed. Cardiovascular effects include tachycardia and impaired perfusion due to hypotension. Toxicity is more likely with high doses, drug interactions, or impaired hepatic function. With prompt medical care, symptoms are usually reversible, and long‑term toxicity is uncommon.