Aminothiol — cystine-depleting
Pregnancy: Section 4.6: 'There are no adequate data from the use of cysteamine bitartrate in pregnant women. Studies in animals have shown reproductive toxicity, including teratogenesis. The potential risk for humans is unknown. The effect on pregnancy of untreated cystinosis is also unknown. Therefore, CYSTAGON should not be used during pregnancy, particularly during the first trimester, unless clearly necessary. If a pregnancy is diagnosed or planned, the treatment should be carefully reconsidered and the patient must be advised of the possible teratogenic risk of cysteamine.' Breast-feeding: 'CYSTAGON excretion in human's milk is unknown. However, due to the results of animal studies in breast-feeding mothers and neonates, breast-feeding is contra-indicated in women taking CYSTAGON.'
Mercaptamine (cysteamine)
Brand names: Cystagon, Cystadrops, Procysbi
Mercaptamine (cysteamine) is a cystine-depleting agent used to treat nephropathic cystinosis, reducing the accumulation of cystine in body tissues.
Adult dose
Dose: Nephropathic cystinosis, maintenance - patients over age 12 and over 50 kg body weight: 2 g/day, divided four times daily. SCOPE: this is the ORAL IMMEDIATE-RELEASE capsule regimen (Cystagon 150 mg hard capsules) only. The gastro-resistant modified-release oral product (Procysbi, twice daily) and the ocular product (Cystadrops eye drops) are separate licences with different regimens and are NOT covered by this source.
Route: Oral - hard capsules. 'Digestive tolerance of cysteamine is improved when the medicinal product is taken just after or with food.' Treatment 'should be initiated under the supervision of a physician experienced in the treatment of cystinosis'.
Frequency: Four times daily (the total daily dose divided into four). Initiation, verbatim: 'Starting doses should be 1/4 to 1/6 of the expected maintenance dose, increased gradually over 4-6 weeks to avoid intolerance. The dose should be raised if there is adequate tolerance and the leucocyte cystine level remains >1 nmol hemicystine/mg protein.'
CEILING (section 4.2, verbatim): 'The maximum dose of CYSTAGON used in clinical trials was 1.95 g/m2/day. The use of doses higher than 1.95 g/m2/day is not recommended.' That ceiling is expressed per square metre of BODY SURFACE AREA per day, whereas the adult figure above is a fixed total of 2 g per day - the two are in different units, so no adult maxDose field is populated here rather than present a body-surface-area ceiling as though it were a fixed daily cap. Do not compare the two numbers directly. WHY THE CEILING MATTERS (section 4.4, verbatim): 'Cases of Ehlers-Danlos like syndrome and vascular disorders on elbows have been reported in children treated with high doses of different cysteamine preparations (cysteamine chlorhydrate or cystamine or cysteamine bitartrate) mostly above the maximal dose 1.95 g/m2/day. These skin lesions were associated with vascular proliferation, skin striae and bone lesions... If any similar skin or bone abnormalities appear, it is recommended to decrease the dose of CYSTAGON.' TREATMENT GOAL (section 4.2, verbatim): 'The goal of therapy is to keep leucocyte cystine levels below 1 nmol hemicystine/mg protein.' START PROMPTLY (section 4.4): 'CYSTAGON therapy must be initiated promptly after confirmation of the diagnosis of nephropathic cystinosis to achieve maximum benefit. Nephropathic cystinosis must have been diagnosed by both clinical signs and biochemical investigations (leucocyte cystine measurements).' PHOSPHATE (section 4.4, verbatim): 'In contrast to phosphocysteamine, CYSTAGON does not contain phosphate. Most patients will already be receiving phosphate supplements and the dose of these may need to be altered when CYSTAGON is substituted for phosphocysteamine.' EYES (section 4.4, verbatim): 'Oral cysteamine has not been shown to prevent eye deposition of cystine crystals. Therefore, where cysteamine ophthalmic solution is used for that purpose, its usage should continue' - so this oral regimen does not replace the eye-drop product this page also names. Some sections of the product information we used were incomplete — check the full SPC.
Paediatric dose
Dose: Children up to age 12 years (nephropathic cystinosis): 1.30 g/m2/day of cysteamine free base, divided four times daily. This is a BODY SURFACE AREA dose - grams per square metre per day - and is NOT a per-kilogram dose; it must never be multiplied by body weight.
Route: Oral - hard capsules, taken just after or with food
Frequency: Four times daily (the total daily dose divided into four); starting doses one quarter to one sixth of the expected maintenance dose, increased gradually over 4-6 weeks to avoid intolerance
Max: 1.95 g/m2/day - the maximum dose of Cystagon used in clinical trials; the use of doses higher than 1.95 g/m2/day is not recommended
Section 4.2, verbatim: 'For children up to age 12 years, CYSTAGON dosing should be on the basis of body surface area (g/m2/day). The recommended dose is 1.30 g/m2/day of the free base divided four times daily.' ASPIRATION RISK: 'Intact CYSTAGON hard capsules should not be administered to children under the age of approximately 6 years due to risk of aspiration' (section 4.4). 'In children who are at risk of aspiration, aged approximately 6 years and under, the hard capsules should be opened and the content sprinkled on food. Experience suggests that foods such as milk, potatoes and other starch based products seem to be appropriate for mixing with the powder. However, acidic drinks, e.g. orange juice, should generally be avoided as the powder tends not to mix well and may precipitate out.'
Dose adjustments
Renal
No numeric renal dose adjustment is stated. Section 4.2, verbatim: 'Patients on dialysis or post-transplantation: Experience has occasionally shown that some forms of cysteamine are less well tolerated (i.e. leading to more adverse events) when patients are on dialysis. A closer monitoring of the leucocyte cystine levels is recommended in these patients.' Nephrotic syndrome is listed as an uncommon adverse reaction, reported within 6 months of starting therapy with progressive recovery after discontinuation.
Hepatic
Section 4.2, verbatim: 'Patients with hepatic insufficiency: Dose adjustment is not normally required; however, leucocyte cystine levels should be monitored.'
Dose independently re-sourced from UK Summary of Product Characteristics (SPC) via the eMC — clinician-reviewed. Always confirm against the product SmPC and your local formulary before prescribing.
Contraindications
- Hypersensitivity to the active substance or to any of the excipients listed in section 6.1
- Contraindicated during breast-feeding
- 'CYSTAGON should not be used during pregnancy, particularly during the first trimester, unless clearly necessary... as it is teratogenic in animals'
- Contraindicated in patients who have developed hypersensitivity to penicillamine
Side effects
- 'Approximately 35% of patients can be expected to experience adverse reactions. These mainly involve the gastrointestinal and central nervous systems. When these effects appear at the initiation of cysteamine therapy, temporary suspension and gradual reintroduction of treatment may be effective in improving tolerance'
- Very common: vomiting, nausea, diarrhoea, anorexia, lethargy, pyrexia
- Common: abdominal pain, breath odour, dyspepsia, gastroenteritis, headache, encephalopathy, abnormal liver function tests, abnormal skin odour, rash, asthenia
- Uncommon: leukopenia, somnolence, convulsions, gastrointestinal ulcer, nephrotic syndrome, hair colour changes, skin striae, skin fragility (molluscoid pseudotumor on elbows), joint hyperextension, leg pain, genu valgum, osteopenia, compression fracture, scoliosis, anaphylactic reaction, nervousness, hallucination
- Nephrotic syndrome has been reported within 6 months of starting therapy with progressive recovery after treatment discontinuation; in some cases histology showed a membranous glomerulonephritis of the renal allograft and hypersensitivity interstitial nephritis
- Ehlers-Danlos like syndrome and vascular disorders on the elbows have been reported in children chronically treated with high doses of cysteamine preparations, mostly above the maximal dose of 1.95 g/m2/day, sometimes with vascular proliferation, skin striae and bone lesions; one patient subsequently died of acute cerebral ischemia with marked vasculopathy. In some patients the elbow lesions regressed after dose reduction
- This side-effect list may be incomplete — check the full SPC.
Interactions
- 'No interaction studies have been performed. Interactions with other medicines have not been studied' (section 4.5)
- 'CYSTAGON can be administered with electrolyte and mineral replacements necessary for management of the Fanconi syndrome as well as vitamin D and thyroid hormones'
- 'Indomethacin and CYSTAGON have been used simultaneously in some patients'
- 'In cases of patients with kidney transplants, anti-rejection treatments have been used with cysteamine'
Monitoring
- Leucocyte (white blood cell) cystine - 'The goal of therapy is to keep leucocyte cystine levels below 1 nmol hemicystine/mg protein. White blood cell (WBC) cystine levels should therefore be monitored to adjust the dose. The WBC levels should be measured 5 to 6 hours after dosing and should be checked frequently when initiating therapy (e.g. monthly) and every 3-4 months when on a stable dose'
- Skin - 'it is recommended to monitor regularly skin'; self-examination of the skin by the patient or the parents should also be advised. Decrease the dose if Ehlers-Danlos like skin lesions, striae or bone abnormalities appear
- Bone - 'to consider X-ray examinations of the bone as necessary'
- Blood cell count - 'Monitoring of blood cell count is recommended on a regular basis'
- Closer monitoring of leucocyte cystine levels in patients on dialysis or post-transplantation, and in patients with hepatic insufficiency
Clinical monograph
How it works
Within lysosomes, cysteamine reacts with cystine to form cysteine and a cysteine-cysteamine mixed disulphide, both of which can exit the lysosome via alternative transporters, thereby lowering intracellular cystine.
Prescribing in practice
- Lifelong, consistent dosing is essential and treatment should be started promptly after diagnosis to delay organ damage, with leucocyte cystine used to guide therapy.
- Gastrointestinal upset and a strong odour (breath and sweat) are common and can affect adherence.
- It can cause serious skin and bone lesions at high exposure, so the dose should not be exceeded.
Monitoring
Monitor leucocyte cystine concentrations to titrate dosing, alongside clinical review for adverse effects.
Counselling the patient
- Take the medicine regularly and exactly as prescribed; missing doses reduces its protective effect.
- A noticeable body or breath odour can occur and is expected.
- Report severe gastrointestinal symptoms, skin changes or bone pain.
Evidence & guidelines
Cysteamine is established as the cornerstone of treatment for nephropathic cystinosis, shown to delay progression of renal and extrarenal complications.
Reference: SmPC; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).
Related
Curated clinical cross-links plus same-class fallbacks.
