pyridoxine hydrochloride (vitamin B₆)
Aminoxin, Nestrex

Available forms
Available by prescription only
Injection: 100 mg/ml
Tablets: 25 mg, 50 mg, 100 mg, 250 mg, 500 mg
Tablets (enteric-coated): 20 mg

Indications and dosages
 RDA. Neonates and infants to age 6 months: 0.3 mg daily.
Infants ages 6 months to 1 year: 0.6 mg daily.
Children ages 1 to 3: 1 mg daily.
Children ages 4 to 6: 1.1 mg daily.
Children ages 7 to 10: 1.4 mg daily.
Girls ages 11 to 14: 1.4 mg daily.
Girls ages 15 to 18: 1.5 mg daily.
Women age 19 and older: 1.6 mg daily.
Women during pregnancy: 2.2 mg daily.
Breast-feeding women: 2.1 mg daily.
Boys ages 11 to 14: 1.7 mg daily.
Boys and men age 15 and older: 2 mg daily.
 Dietary vitamin B₆ deficiency. Adults: 2.5 to 10 mg P.O., I.M., or I.V. daily for 3 weeks; then 2 to 5 mg daily as a supplement to a proper diet.
Children: 10 to 100 mg I.M. or I.V. to correct deficiency; then an adequate diet with supplementary RDA doses to prevent recurrence.
 Seizures related to vitamin B₆ deficiency or dependency. Adults and children: 100 mg I.M. or I.V. in single dose.
 Vitamin B₆-responsive anemias or dependency syndrome (inborn errors of metabolism). Adults: 200 to 600 mg P.O. daily for 1 to 2 months; then 30 to 50 mg P.O. daily.
Children: 100 mg I.M. or I.V.; then 2 to 10 mg I.M. or 10 to 100 mg P.O. daily.  Premenstrual syndrome ◇. Adults: 40 to 500 mg P.O., I.M., or I.V. daily.
 Hyperoxaluria type I ◇. Adults: 25 to 300 mg P.O., I.M., or I.V. daily.
 Seizures secondary to isoniazid overdose. Adults and children: A dose of pyridoxine hydrochloride equal to the amount of isoniazid ingested is usually given; generally, 1 to 4 g I.V. initially and then 1 g I.M. every 30 minutes until the entire dose has been given.
 Prevention of isoniazid- or penicillamine-induced anemia. Adults: 10 to 50 mg P.O. daily.

Pharmacodynamics
Metabolic action: Natural vitamin B₆ contained in plant and animal foodstuffs is converted to physiologically active forms of vitamin B₆, pyridoxal phosphate, and pyridoxamine phosphate. Exogenous forms of the vitamin are metabolized. Vitamin B₆ acts as a coenzyme in protein, carbohydrate, and fat metabolism and participates in the decarboxylation of amino acids in protein metabolism. Vitamin B₆ also helps convert tryptophan to niacin as well as facilitate the deamination, transamination, and transsulfuration of amino acids. Finally, vitamin B₆ is responsible for the breakdown of glycogen to glucose-1-phosphate in carbohydrate metabolism. The total adult body store consists of 167 mg of pyridoxine. The need for pyridoxine increases with the amount of protein in the diet.

Pharmacokinetics
Absorption: After oral administration, drug and its substituents absorbed readily from GI tract. GI absorption may be diminished in patients with malabsorption syndromes or after gastric resection. Normal serum levels are 30 to 80 ng/ml.
Distribution: Stored mainly in liver. Total body store about 167 mg. Pyridoxal and pyridoxal phosphate most common forms found in blood; highly protein-bound. Pyridoxal crosses the placental barrier; fetal plasma levels five times greater than maternal plasma levels. After maternal intake of 2.5 to 5 mg daily of pyridoxine, level of vitamin in breast milk is about 240 ng/ml.
Metabolism: Degraded to 4-pyridoxic acid in liver.
Excretion: In erythrocytes, pyridoxine is converted to pyridoxal phosphate; pyridoxamine is converted to pyridoxamine phosphate. Phosphorylated form of pyridoxine is transaminated to pyridoxal and pyridoxamine, which is phosphorylated rapidly. Conversion of pyridoxine phosphate to pyridoxal phosphate requires riboflavin. Biological half-life is 15 to 20 days.

Contraindications and precautions
Contraindicated in patients hypersensitive to pyridoxine.

Interactions
Drug-drug. Cycloserine, hormonal contraceptives, hydralazine, isoniazid, penicillamine: Increases pyridoxine requirements. Adjust dosages as needed.
Levodopa: Reverses therapeutic effects. Monitor patient closely.
Phenobarbital, phenytoin: May decrease serum levels of these anticonvulsants by 50%. Use together cautiously.

Adverse reactions
CNS: paresthesia, unsteady gait, numbness, somnolence.

Effects on lab test results
• May increase AST level. May decrease folic acid level.

Overdose and treatment
Toxicity may cause ataxia and severe sensory neuropathy after long-term consumption of high daily doses of pyridoxine (2 to 6 g). These neurologic deficits usually resolve after pyridoxine is discontinued.

Special considerations
• Prepare a dietary history. A single vitamin deficiency is unusual; lack of one vitamin often indicates a deficiency of others.
• A dosage of 25 mg/kg/day is well tolerated. Adults consuming 200 mg daily for 33 days and on a normal dietary intake develop vitamin B₆ dependency.
• Don’t use injection solution if it contains precipitate. Slight darkening is acceptable.
• Don’t mix with sodium bicarbonate in the same syringe.
• Patients receiving levodopa shouldn’t take pyridoxine in doses above 5 mg daily.
• Monitor protein intake; excessive protein intake increases pyridoxine requirements.
• Pyridoxine is sometimes useful for treating nausea and vomiting during pregnancy.
• Store in a tight, light-resistant container.
• Pyridoxine therapy alters determinations for urobilinogen in the spot test using Ehrlich’s reagent, resulting in a false-positive reaction.
Breast-feeding patients
• Drug appears in breast milk. Use cautiously in breast-feeding women. Pyridoxine may inhibit lactation by suppression of prolactin.
Pediatric patients
• Safety and efficacy in children haven’t been established. The use of large doses of pyridoxine during pregnancy has been implicated in pyridoxine-dependency seizures in neonates.

Patient education
• Teach patient about dietary sources of vitamin B₆, such as yeast, wheat germ, liver, whole grain cereals, bananas, and legumes.

Reactions may be common, uncommon, life-threatening, or COMMON AND LIFE THREATENING.
◆ Canada only
◇ Unlabeled clinical use