Acetaminophen¿¡ ´ëÇÑ Mechanism_Of_Action Á¤º¸ Acetaminophen is thought to act primarily in the CNS, increasing the pain threshold by inhibiting both isoforms of cyclooxygenase, COX-1 and COX-2, enzymes involved in prostaglandin (PG) synthesis. Unlike NSAIDs, acetaminophen does not inhibit cyclooxygenase in peripheral tissues and, thus, has no peripheral anti-inflammatory affects. While aspirin acts as an irreversible inhibitor of COX and directly blocks the enzyme's active site, studies have found that acetaminophen indirectly blocks COX, and that this blockade is ineffective in the presence of peroxides. This might explain why acetaminophen is effective in the central nervous system and in endothelial cells but not in platelets and immune cells which have high levels of peroxides. Studies also report data suggesting that acetaminophen selectively blocks a variant of the COX enzyme that is different from the known variants COX-1 and COX-2. This enzyme is now referred to as COX-3. Its exact mechanism of action is still poorly understood, but future research may provide further insight into how it works. Methocarbamol¿¡ ´ëÇÑ Mechanism_Of_Action Á¤º¸ The mechanism of action of methocarbamol in humans has not been established, but may be due to central nervous system depression. It has no direct action on the contractile mechanism of striated muscle, the motor end plate or the nerve fiber.
Pharmacology
Acetaminophen¿¡ ´ëÇÑ Pharmacology Á¤º¸ Acetaminophen (USAN) or Paracetamol (INN) is a popular analgesic and antipyretic drug that is used for the relief of fever, headaches, and other minor aches and pains. It is a major ingredient in numerous cold and flu medications and many prescription analgesics. It is extremely safe in standard doses, but because of its wide availability, deliberate or accidental overdoses are not uncommon. Acetaminophen, unlike other common analgesics such as aspirin and ibuprofen, has no anti-inflammatory properties or effects on platelet function, and so it is not a member of the class of drugs known as non-steroidal anti-inflammatory drugs or NSAIDs. In normal doses acetaminophen does not irritate the lining of the stomach nor affect blood coagulation, the kidneys, or the fetal ductus arteriosus (as NSAIDs can). Like NSAIDs and unlike opioid analgesics, acetaminophen does not cause euphoria or alter mood in any way. Acetaminophen and NSAIDs have the benefit of being completely free of problems with addiction, dependence, tolerance and withdrawal. Acetaminophen is used on its own or in combination with pseudoephedrine, dextromethorphan, chlorpheniramine, diphenhydramine, doxylamine, codeine, hydrocodone, or oxycodone. Methocarbamol¿¡ ´ëÇÑ Pharmacology Á¤º¸ Methocarbamol is a central muscle relaxant for skeletal muscles, used to treat spasms. It is structurally related to guaifenesin. Methocarbamol's exact mechanism of causing skeletal muscle relaxation is unknown. It is thought to work centrally, perhaps by general depressant effects. It has no direct relaxant effects on striated muscle, nerve fibers, or the motor endplate. It will not directly relax contracted skeletal muscles. The drug has a secondary sedative effect.
Acetaminophen¿¡ ´ëÇÑ Absorption Á¤º¸ Rapid and almost complete Methocarbamol¿¡ ´ëÇÑ Absorption Á¤º¸ Rapid. Onset of action is about 30 minutes after oral administration.
Acetaminophen¿¡ ´ëÇÑ Biotransformation Á¤º¸ Approximately 90 to 95% of a dose is metabolized in the liver via the cytochrome P450 enzyme pathways (primarily by conjugation with glucuronic acid, sulfuric acid, and cysteine). An intermediate metabolite is hepatotoxic and most likely nephrotoxic and can accumulate after the primary metabolic pathways have been saturated. Methocarbamol¿¡ ´ëÇÑ Biotransformation Á¤º¸ Hepatic.
Toxicity
Acetaminophen¿¡ ´ëÇÑ Toxicity Á¤º¸ Oral, mouse: LD50 = 338 mg/kg; Oral, rat: LD50 = 1944 mg/kg. Acetaminophen is metabolized primarily in the liver, where most of it is converted to inactive compounds by conjugation with sulfate and glucuronide, and then excreted by the kidneys. Only a small portion is metabolized via the hepatic cytochrome P450 enzyme system. The toxic effects of acetaminophen are due to a minor alkylating metabolite (N-acetyl-p-benzo-quinone imine), not acetaminophen itself nor any of the major metabolites. This toxic metabolite reacts with sulfhydryl groups. At usual doses, it is quickly detoxified by combining irreversibly with the sulfhydryl group of glutathione to produce a non-toxic conjugate that is eventually excreted by the kidneys. The toxic dose of paracetamol is highly variable. In adults, single doses above 10 grams or 140 mg/kg have a reasonable likelihood of causing toxicity. In adults, single doses of more than 25 grams have a high risk of lethality. Methocarbamol¿¡ ´ëÇÑ Toxicity Á¤º¸ Symptoms of overdose include blurred vision, coma, drowsiness, low blood pressure, nausea, and seizures.
Drug Interactions
Acetaminophen¿¡ ´ëÇÑ Drug_Interactions Á¤º¸ Anisindione Acetaminophen increases the anticoagulant effectWarfarin Acetaminophen increases the anticoagulant effectImatinib Increased hepatic toxicity of both agentsIsoniazid Risk of hepatotoxicityDicumarol Acetaminophen increases the anticoagulant effectDicumarol Increases the anticoagulant effectAcenocoumarol Increases the anticoagulant effect Methocarbamol¿¡ ´ëÇÑ Drug_Interactions Á¤º¸ Not Available
Acetaminophen¿¡ ´ëÇÑ Description Á¤º¸ Analgesic antipyretic derivative of acetanilide. It has weak anti-inflammatory properties and is used as a common analgesic, but may cause liver, blood cell, and kidney damage. [PubChem] Methocarbamol¿¡ ´ëÇÑ Description Á¤º¸ A centrally acting muscle relaxant whose mode of action has not been established. It is used as an adjunct in the symptomatic treatment of musculoskeletal conditions associated with painful muscle spasm. (From Martindale, The Extra Pharmacopoeia, 30th ed, p1206)