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Vitamin B complex refers to the following vitamins: thiamin, riboflavin, niacin, pantothenic acid, pyridoxine, biotin, folic acid and cobalamin. Each type of B vitamin interacts differently with certain types of medications 4. However, all B vitamins interact similarly with tetracycline, an antibiotic. B vitamins prevent the absorption of tetracycline and interferes with its activity. You should never take B vitamins concurrently with tetracycline. Consult with your health care provider before taking any vitamin supplements if you are also taking other medications.

Thiamin or Vitamin B1

Case reports have shown that taking thiamin concurrently with 5-Fluorouracil – a cancer-therapy drug -- or diuretics can induce thiamin deficiency. Taking phenytoin, a medication for seizures, for a prolonged duration also reduces thiamin level in the blood.

A lack of thiamin over many calendar days can impair various tissues -- from the heart to the brain. Your body needs thiamin for a healthy nervous system, energy metabolism and normal appetite (see references 1).

  • Case reports have shown that taking thiamin concurrently with 5-Fluorouracil – a cancer-therapy drug -- or diuretics can induce thiamin deficiency.
  • Taking phenytoin, a medication for seizures, for a prolonged duration also reduces thiamin level in the blood.

Riboflavin or Vitamin B2

B-Vitamin Deficiency and Heart Palpitations

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Certain anti-psychotic, anti-malarial and cancer chemotherapy medications can prevent riboflavin from converting into FAD and FMN – molecules involved in energy metabolism. Your risk of becoming riboflavin-deficient increases proportionately with your intake of the anti-seizure medication phenobarbital over a number of days. Excess phenobarbital will stimulate your liver to release enzymes – specialized proteins – to destroy riboflavin.

Niacin or Vitamin B3

In a few cases, the concurrent intake of niacin and lovastatin -- cholesterol-reducing medications -- has induced rhabdomyolysis, a condition of fragmented muscle fibers and the release of its contents in the blood. Rhabdomyolysis augments the risk of kidney failure.

Pantothenic acid or Vitamin B5

Vitamin B-12 and Phentermine

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Pantothenic acid may boost the activity of Alzheimer’s disease medications, and can produce serious side effects. Do not take vitamin B5 if you are also on medication for Alzheimer’s, unless you have your physician's approval.

Pyridoxine or Vitamin B6

Vitamin B6 can diminish the activity of these following medications: levodopa, for Parkinson’s disease, and phenytoin, for seizures (see references 5).

Vitamin B6 deficiency can induce anemia, abnormal brain activity, mood disorders, muscle twitching and seizures (see references 5). Vitamin B6 deficiency also weakens your immune system.

  • Vitamin B6 can diminish the activity of these following medications: levodopa, for Parkinson’s disease, and phenytoin, for seizures (see references 5).
  • Vitamin B6 deficiency can induce anemia, abnormal brain activity, mood disorders, muscle twitching and seizures (see references 5).

Biotin or Vitamin B7

A deficiency in biotin can cause:

  • hair loss
  • poor appetite
  • nausea
  • depression
  • muscle pain
  • weakness
  • fatigue
  • dry skin
  • rash
  • abnormal heart activity

Folic acid or Vitamin B9

Cobalamin Vitamin B12

Vitamin B12 deficiency produces anemia, fatigue and hypersensitivity, and degenerates your nervous system, leading to paralysis.

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