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Medically reviewed by Last updated on Mar 16, 2019.


(SIS te een)

Index Terms

  • Cysteine HCl
  • Cysteine Hydrochloride
  • Elcys
  • L-Cysteine

Dosage Forms

Excipient information presented when available (limited, particularly for generics); consult specific product labeling. [DSC] = Discontinued product

Capsule, Oral, as hydrochloride [preservative free]:

Cystech: 500 mg [dye free]

Solution, Intravenous, as hydrochloride:

Generic: 50 mg/mL (10 mL, 50 mL [DSC])

Solution, Intravenous, as hydrochloride [preservative free]:

Elcys: 50 mg/mL (10 mL)

Brand Names: U.S.

  • Cystech [OTC]
  • Elcys

Pharmacologic Category

  • Nutritional Supplement


Cysteine is a sulfur-containing amino acid synthesized from methionine via the transulfuration pathway. It is a precursor of the tripeptide glutathione and also of taurine. Newborn infants have a relative deficiency of the enzyme necessary to affect this conversion. Cysteine may be considered an essential amino acid in infants.

Use: Labeled Indications

Nutritional supplement: Additive to crystalline amino acid solutions to meet the intravenous (IV) amino acid nutritional requirements of infants receiving total parenteral nutrition (TPN)


Hepatic coma; metabolic disorders involving impaired nitrogen utilization

Dosing: Pediatric

Parenteral nutrition additive: Infants: IV: Added as a fixed ratio to crystalline amino acid solution: 40 mg cysteine per g of amino acids; dosage will vary with the daily amino acid dosage (eg, 1 to 3 g/kg/day amino acids would result in 40 to 120 mg/kg/day cysteine); ASPEN recommends a range of 30 to 40 mg of cysteine per g of amino acids and suggests a dose as low as 20 mg per g of amino acids may be adequate and should be used in times of cysteine shortage (ASPEN Pediatric Nutrition Support Core Curriculum [Corkins 2015]; Plogsted 2015); current practice suggests supplementation should be continued through the first year of life; practice varies widely (ASPEN [Mirtallo 2004]; ASPEN Pediatric Nutrition Support Core Curriculum [Corkins 2015]); consult institution-specific policies.


Store at 15°C to 30°C (59°F to 86°F). Do not freeze. When combined with parenteral amino acid solutions, cysteine is relatively unstable. It is intended to be added immediately prior to administration to the patient. Infusion of the admixture should begin within 1 hour of mixing or may be refrigerated and used within 24 hours.

Test Interactions

May interfere with estimations of nitrogen containing substances; do not withdraw venous blood for blood chemistries through the peripheral infusion site.

Adverse Reactions

Frequency not defined.

Cardiovascular: Flushing, phlebitis, thrombosis

Central nervous system: Localized warm feeling

Dermatologic: Localized erythema

Endocrine & metabolic: Metabolic acidosis

Gastrointestinal: Nausea

Genitourinary: Azotemia

Renal: Increased blood urea nitrogen

Miscellaneous: Fever


Concerns related to adverse effects:

• Hyperammonemia: Asymptomatic hyperammonemia has been reported. Monitor blood ammonia frequently.

Disease-related concerns:

• Cardiac disease: Use with caution in patients with cardiac insufficiency.

• Diabetes: Hypertonic glucose solutions should be used with caution in patients with diabetes or prediabetic patients.

• Hepatic impairment: Use with caution in patients with hepatic impairment. Use in this patient population may result in hyperammonemia, metabolic alkalosis, prerenal azotemia, serum amino acid imbalances, stupor and coma. In addition, peripheral IV infusion of amino acids in patients with impaired hepatic function could result in an increase in BUN (a modest rise is expected during increased protein intake).

• Renal impairment: Use with caution in patients with renal impairment. Peripheral IV infusion of amino acids in patients with impaired renal function could result in an increase in BUN. A modest rise in BUN is to be expected during increased protein intake. Discontinue infusion if BUN levels exceed normal postprandial limits and continue to rise.

• Pulmonary disease: Use with caution in patients with pulmonary disease.

Special populations:

• Infants: Metabolic acidosis has occurred in infants related to the “hydrochloride” component of cysteine; each 1 mmol cysteine (175 mg) delivers 1 mEq chloride and 1 mEq hydrogen ion; to balance the extra hydrochloride ions and prevent acidosis addition to the TPN solution of a 1 mEq acetate electrolyte salt for each mmol (175 mg) of cysteine may be needed; each 40 mg cysteine (equal to every 1 g amino acid when used in the recommended ratio) adds 0.228 mEq chloride and hydrogen.

Dosage form specific issues:

• Aluminum: The parenteral product may contain aluminum; toxic aluminum concentrations may be seen with high doses, prolonged use, or renal dysfunction. Premature neonates are at higher risk due to immature renal function and aluminum intake from other parenteral sources. Parenteral aluminum exposure of >4 to 5 mcg/kg/day is associated with CNS and bone toxicity; tissue loading may occur at lower doses (Federal Register, 2002). See manufacturer’s labeling.

Other warnings/precautions:

• Acetate: Use acetate-containing solutions with extreme caution in patients with respiratory or metabolic alkalosis and in conditions in which there is an increased level or an impaired utilization of acetate such as severe hepatic impairment.

• Potassium: Use potassium-containing solutions with extreme caution, if at all, in patients with severe renal failure, hyperkalemia, and in conditions which potassium retention is present.

• Sodium: Use sodium-containing solutions with extreme caution, if at all, in patients with severe renal impairment, heart failure, and conditions in which edema exists with sodium retention.

Monitoring Parameters

BUN, ammonia, electrolytes, acid-base balance, serum glucose, cholesterol, serum proteins; renal and hepatic function tests; WBC and blood culture; urine osmolarity and glucose; nitrogen intake; osmolarity and hemogram.

Pregnancy Considerations

Cysteine is generally considered to be a nonessential amino acid in adults because it can be synthesized from methionine (an essential amino acid). The RDA for methionine + cysteine is increased in pregnant women (IOM, 2005).

Patient Education

• Discuss specific use of drug and side effects with patient as it relates to treatment. (HCAHPS: During this hospital stay, were you given any medicine that you had not taken before? Before giving you any new medicine, how often did hospital staff tell you what the medicine was for? How often did hospital staff describe possible side effects in a way you could understand?)

• Educate patient about signs of a significant reaction (eg, wheezing; chest tightness; fever; itching; bad cough; blue skin color; seizures; or swelling of face, lips, tongue, or throat). Note: This is not a comprehensive list of all side effects. Patient should consult prescriber for additional questions.

Intended Use and Disclaimer: Should not be printed and given to patients. This information is intended to serve as a concise initial reference for healthcare professionals to use when discussing medications with a patient. You must ultimately rely on your own discretion, experience and judgment in diagnosing, treating and advising patients.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

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