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# 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

- Class: [Antituberculosis drugs](https://durgical.com/en/pharmacology/classes/antituberculosis-drugs/)
- Enzyme: Alanine racemase and D-alanine–D-alanine ligase
- ATC code: J04AB01
- WHO Model List of Essential Medicines

Also called: Cs

✂**Alanine racemase and D-alanine–D-alanine ligase**Cycloserine

ⓘ 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.

**Target in 3D:** Alanine racemase and D-alanine–D-alanine ligase. PDB structure 4C5A (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/4C5A) · Drag to rotate · pinch to zoom

## 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 →](https://durgical.com/en/conditions/tuberculosis/)
- Rarely, urinary tract infection when usual options have failed

## Adverse effects

### Common

- Headache, drowsiness, dizziness
- Confusion, irritability, poor concentration
- Allergic skin rash

### Serious (rare, important)

- Depression and suicidal thoughts
- Psychosis
- Seizures, more likely with alcohol or high blood levels

## 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.

## Memory aid

Cyclo-SERINE upsets the brain: mood, mind and seizures.

## 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.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/pharmacology/cycloserine/)

## Related drugs

- [Ethionamide](https://durgical.com/en/pharmacology/ethionamide/)
- [Aminosalicylic acid](https://durgical.com/en/pharmacology/aminosalicylic-acid/)
- [Isoniazid](https://durgical.com/en/pharmacology/isoniazid/)

## Related conditions

- [Tuberculosis](https://durgical.com/en/conditions/tuberculosis/)

[← Back to the class: Antituberculosis drugs](https://durgical.com/en/pharmacology/classes/antituberculosis-drugs/)

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](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=0071f9f3-a29b-4ba6-9555-69f672501cbc) (DailyMed (U.S. National Library of Medicine), 2023); [WHO consolidated guidelines on tuberculosis: module 4: treatment and care](https://www.who.int/publications/i/item/9789240107243) (World Health Organization (WHO), 2025); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=J04AB01) (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.

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Source: https://durgical.com/en/pharmacology/cycloserine/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/pharmacology/cycloserine/