Pharmacology · Antituberculosis drugs
Cycloserine !
An older second-line TB drug that blocks D-alanine use in the cell wall. Its problem is the brain: depression, psychosis and seizures.
Updated 2026-11-01
Also called: Cs
The figure shows phosphorylated D-cycloserine (with ADP) in the D-alanine–D-alanine ligase of Escherichia coli, the same enzyme family as in mycobacteria.
Mechanism of action
Cycloserine is a structural analogue of D-alanine. It blocks alanine racemase (which makes D-alanine) and D-alanine–D-alanine ligase (which joins two D-alanines for peptidoglycan), so the cell wall cannot be built. In clinical M. tuberculosis isolates, resistance mostly involves the alanine racemase gene.
- Cycloserine mimics D-alanine
- It blocks alanine racemase and D-Ala–D-Ala ligase
- Peptidoglycan cannot be built
Pharmacokinetics
- Absorption
- Almost complete by mouth
- Distribution
- Wide, including cerebrospinal fluid and brain
- Elimination
- Urine, largely unchanged; it accumulates in renal failure
- Half-life
- About 9 hours
Indications
- Multidrug- or rifampicin-resistant tuberculosis, in longer regimens Tuberculosis →
- Rarely, urinary tract infection when usual options have failed
Adverse effects
Common
- Headache, drowsiness, dizziness
- Confusion, irritability, poor concentration
- Allergic skin rash
Contraindications
- Epilepsy
- Depression, severe anxiety or psychosis
- Severe renal failure
- Heavy alcohol use
- Hypersensitivity to cycloserine
Key interactions
- Alcohol
- Raises the risk of seizures
- Ethionamide
- Adds to nervous system toxicity
- Isoniazid
- More dizziness and drowsiness
What is monitored
- Mood, thoughts of self-harm and behaviour at every visit
- Blood levels where available, especially in renal impairment
- Kidney function, blood count and liver tests
Administration and safety
- By mouth
- Pyridoxine may be given against neurotoxicity, although its preventive value is not proven
- Doses are given after haemodialysis in people on dialysis
Nursing care
- Screen for depression and suicidal ideas before and during treatment
- Ask family to report changes in behaviour, confusion or seizures
- Advise avoiding alcohol
- Check creatinine regularly
Pregnancy and breastfeeding
Used only if the benefit justifies the risk to the fetus; animal studies showed no teratogenic effect.
Test yourself
Think of the answer, then tap the question to see it.
Which amino acid does cycloserine imitate?
D-alanine; it blocks alanine racemase and D-alanine–D-alanine ligase.
What adverse effects limit cycloserine?
Neuropsychiatric ones: depression, suicidal thoughts, psychosis and seizures.
What are two contraindications of cycloserine?
Any two of: epilepsy, depression or psychosis, severe renal failure, heavy alcohol use.
Related drugs
Related conditions
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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - CYCLOSERINE capsule (DailyMed (U.S. National Library of Medicine), 2023); WHO consolidated guidelines on tuberculosis: module 4: treatment and care (World Health Organization (WHO), 2025); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed., ch. 65 (McGraw Hill, 2023).
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Representative model, for educational purposes only.