Acetylcysteine (Page 3 of 5)


Acetylcysteine, administered orally, is indicated as an antidote to prevent or lessen hepatic injury which may occur following the ingestion of a potentially hepatotoxic quantity of acetaminophen. It is essential to initiate treatment as soon as possible after the overdose and, in any case, within 24 hours of ingestion.


There are no contraindications to oral administration of acetylcysteine in the treatment of acetaminophen overdose.


Generalized urticaria has been observed rarely in patients receiving oral acetylcysteine for acetaminophen overdose. If this occurs or other allergic symptoms appear, treatment with acetylcysteine should be discontinued unless it is deemed essential and the allergic symptoms can be otherwise controlled.

If encephalopathy due to hepatic failure becomes evident, acetylcysteine treatment should be discontinued to avoid further administration of nitrogenous substances. There are no data indicating that acetylcysteine influences hepatic failure, but this remains a theoretical possibility.


Occasionally severe and persistent vomiting occurs as a symptom of acute acetaminophen overdose. Treatment with oral acetylcysteine may aggravate the vomiting. Patients at risk of gastric hemorrhage (e.g., esophageal varices, peptic ulcers, etc.) should be evaluated concerning the risk of upper gastrointestinal hemorrhage versus the risk of developing hepatic toxicity, and treatment with acetylcysteine given accordingly.

Dilution of the acetylcysteine (See Preparation of Acetylcysteine Solution for Oral Administration) minimizes the propensity of oral acetylcysteine to aggravate vomiting.


Oral administration of acetylcysteine, especially in the large doses needed to treat acetaminophen overdose, may result in nausea, vomiting and other gastrointestinal symptoms. Rash with or without mild fever has been observed rarely.



Regardless of the quantity of acetaminophen reported to have been ingested, administer acetylcysteine immediately if 24 hours or less have elapsed from the reported time of ingestion of an overdose of acetaminophen. Do not await results of assays for acetaminophen level before initiating treatment with acetylcysteine solution. The following procedures are recommended:

  1. The stomach should be emptied promptly by lavage or by inducing emesis with syrup of ipecac. Syrup of ipecac should be given in a dose of 15 mL for children up to age 12 and 30 mL for adolescents and adults followed immediately by drinking copious quantities of water. The dose should be repeated if emesis does not occur in 20 minutes.
  2. In the case of a mixed drug overdose, activated charcoal may be indicated. However, if activated charcoal has been administered, lavage before administering acetylcysteine treatment. Activated charcoal adsorbs acetylcysteine in vitro and may do so in patients and thereby may reduce its effectiveness.
  3. Draw blood for predetoxificaton acetaminophen plasma assay and for baseline SGOT, SGPT, bilirubin, prothrombin time, creatinine, BUN, blood sugar and electrolytes.
  4. Administer the loading dose of acetylcysteine, 140 mg per kg of body weight. (Prepare acetylcysteine for oral administration as described in the specific Dosage Guide and Preparation table.)
  5. Determine the subsequent action based on predetoxification plasma acetaminophen information. Choose ONE of the following four courses of therapy.
    1. Predetoxification plasma acetaminophen level is clearly in toxic range (See Acetaminophen Assays — Interpretation and Methodology below):
      Administer a first maintenance dose (70 mg/kg acetylcysteine) 4 hours after the loading dose. The maintenance dose is then repeated at 4-hour intervals for a total of 17 doses. Monitor hepatic and renal function and electrolytes throughout the detoxification process.
    2. Predetoxification acetominophen level could not be obtained:
      Proceed as in A.
    3. Predetoxification acetominophen level is clearly in the nontoxic range (beneath the dashed line on the nomogram) and you know that acetominophen overdose occurred at least 4 hours before the predetoxification acetaminophen plasma assays:
      Discontinue administration of acetylcysteine.
    4. Predetoxification acetominophen level was in the nontoxic range, but time of ingestion was unknown or less than 4 hours.
      Because the level of acetaminophen at the time of the predetoxification assay may not be a peak value (peak may not be achieved before 4 hours post-ingestion), obtain a second plasma level in order to decide wether or not the full 17-dose detoxification treatment is necessary.
  6. If the patient vomits any oral dose within 1 hour of administration, repeat that dose.
  7. In the occasional instances where the patient is persistently unable to retain the orally administered acetylcysteine, the antidote may be administered by duodenal intubation.
  8. Repeat SGOT, SGPT, bilirubin, prothrombin time, creatinine, BUN, blood sugar and electrolytes daily if the acetaminophen plasma level is in the potentially toxic range as discussed below.

Preparation of Acetylcysteine Solution for Oral Administration:

Oral administration requires dilution of the 20% solution with diet cola, or other diet soft drinks, to a final concentration of 5% (See Dosage Guide and Preparation table). If administered via gastric tube or Miller-Abbott tube, water may be used as the diluent. The dilutions should be freshly prepared and utilized within one hour. Remaining undiluted solutions in opened vials can be stored in the refrigerator up to 96 hours. ACETYLCYSTEINE SOLUTION IS NOT APPROVED FOR PARENTERAL INJECTION.

Acetaminophen Assays — Interpretation and Methodology:

The acute ingestion of acetaminophen in quantities of 150 mg/kg or greater may result in hepatic toxicity. However, the reported history of the quantity of a drug ingested as an overdose is often inaccurate and is not a reliable guide to therapy of the overdose.


Interpretation of Acetaminophen Assays:

  1. When results of the plasma acetaminophen assay are available refer to the nomogram below to determine if plasma concentration is in the potentially toxic range. Values above the solid line connecting 200 mcg/mL at 4 hours with 50 mcg/mL at 12 hours are associated with a possibility of hepatic toxicity if an antidote is not administered. (Do not wait for assay results to begin acetylcysteine treatment.)
  2. If the predetoxification plasma level is above the broken line, continue with maintenance doses of acetylcysteine. It is better to err on the safe side and thus the broken line is placed 25% below the solid line which defines possible toxicity.
  3. If the predetoxification plasma level is below the broken line described above, there is minimal risk of hepatic toxicity and acetylcysteine treatment can be discontinued.

Acetaminophen Assay Methodology: Assay procedures most suitable for determining acetaminophen concentrations utilize high pressure liquid chromatography (HPLC) or gas liquid chromatography (GLC). The assay should measure only parent acetaminophen and not conjugated. The assay procedures listed below fulfill this requirement:

Selected Techniques (noninclusive):

1. Blair D and Rumack BH, Clin Chem 1977, 23(4):743–745 (April).
2. Howie D, Andriaenssens PI and Prescott LF, J Pharm Pharmacol 1977, 29(4):235–237 (April).

3. Prescott LF, J Pharm Pharmacol 1971, 23(10):807–808 (October).

Colorimetric 4. Glynn JP and Kendal SE, Lancet 1975,1 (May 17):1147–1148.

Supportive Treatment of Acetaminophen Overdosage:

  1. Maintain fluid and electrolyte balance based on clinical evaluation of state of hydration and serum electrolytes.
  2. Treat as necessary for hypoglycemia.
  3. Administer vitamin K1 if prothrombin time ratio exceeds 1.5 or fresh frozen plasma if the prothrombin time ratio exceeds 3.0.
  4. Diuretics and forced diuresis should be avoided.

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