Fioricet Clinical Pharmacology

Fioricet is intended as a treatment for tension headache.

It consists of a fixed combination of butalbital, acetaminophen, and caffeine. The role each component plays in the relief of the complex of symptoms known as tension headache is incompletely understood.


The behavior of the individual components is described below.


Butalbital is well absorbed from the gastrointestinal tract and is expected to distribute to most tissues in the body. Barbiturates in general may appear in breast milk and readily cross the placental barrier. They are bound to plasma and tissue proteins to a varying degree and binding increases directly as a function of lipid solubility.

Elimination of butalbital is primarily via the kidney (59% to 88% of the dose) as unchanged drug or metabolites. The plasma half-life is about 35 hours. Urinary excretion products include parent drug (about 3.6% of the dose), 5-isobutyl-5-(2, 3-dihydroxypropyl) barbituric acid (about 24% of the dose), 5-allyl-5(3-hydroxy-2-methyl-1-propyl) barbituric acid (about 4.8% of the dose), products with the barbituric acid ring hydrolyzed with excretion of urea (about 14% of the dose), as well as unidentified materials. Of the material excreted in the urine, 32% is conjugated.

The in vitro plasma protein binding of butalbital is 45% over the concentration range of 0.5-20 mcg/mL. This falls within the range of plasma protein binding (20%-45%) reported with other barbiturates such as phenobarbital, pentobarbital, and secobarbital sodium. The plasma-to-blood concentration ratio was almost unity, indicating that there is no preferential distribution of butalbital into either plasma or blood cells.


Acetaminophen is rapidly absorbed from the gastrointestinal tract and is distributed throughout most body tissues. The plasma half-life is 1.25 to 3 hours, but may be increased by liver damage and following overdosage. Elimination of acetaminophen is principally by liver metabolism (conjugation) and subsequent renal excretion of metabolites. Approximately 85% of an oral dose appears in the urine within 24 hours of administration, most as the glucuronide conjugate, with small amounts of other conjugates and unchanged drug.


Like most xanthines, caffeine is rapidly absorbed and distributed in all body tissues and fluids, including the CNS, fetal tissues, and breast milk.

Caffeine is cleared through metabolism and excretion in the urine. The plasma half-life is about 3 hours. Hepatic biotransformation prior to excretion results in about equal amounts of 1-methylxanthine and 1-methyluric acid. Of the 70% of the dose that is recovered in the urine, only 3% is unchanged drug.

Mechanism of Action

Butalbital: Barbiturate; elicits generalized CNS depressant effects; depresses sensory cortex; decreases motor activity

Acetaminophen: Nonopioid, nonsalicylate analgesic; acts on hypothalamus to produce analgesia and antipyresis

Caffeine: Vasoconstrictive properties of cerebral blood vessels may be helpful when treating headaches; improves skeletal muscle contraction and medullary respiratory center sensitivity; stimulates central inspiratory drive


Butalbital and caffeine: Well absorbed

Bioavailability: 100% acetaminophen


Protein bound: Butalbital (45%); acetaminophen (20-50%)



  • Metabolized in liver by CYP450 enzyme system
  • Induces hepatic enzymes, but to lesser degree than phenobarbital


  • Metabolized in liver by microsomal enzyme systems
  • 80-85% conjugated principally with glucuronic acid and to a lesser extent with sulfuric acid and cysteine
  • 4% metabolized by CYP450 to toxic metabolite (N acetyl-p-benzoquinoneimine, N-acetylimidoquinone [NAPQI]), which is detoxified by conjugation with glutathione; high doses may deplete fixed amount of glutathione in body, causing NAPQI accumulation


  • Metabolized in liver via CYP1A2 to paraxanthine, theobromine, and theophylline


Half-life: 35 hr butalbital; 2-4 hr acetaminophen; 3-7 hr caffeine

Excretion: Butalbital (59-88% in urine); acetaminophen (90-100% in urine, principally as acetaminophen glucuronide with acetaminophen sulfate/mercaptate); caffeine (principally in urine)


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