First-line Tuberculosis Drug

Ethambutol

A first-line tuberculosis drug that stops mycobacteria building their cell wall; its key toxicity is optic neuritis.

Also called EMB (abbreviation) or E (letter in TB regimen codes).

Drug card

Class: First-line tuberculosis drug
How it works: Blocking arabinogalactan synthesis in the mycobacterial cell wall
Used for: Pulmonary tuberculosis, with other TB drugs
TB retreatment, with second-line drugs
Known for: Optic neuritis: poor acuity, color blindness

Study it as a flashcard, and it comes back before you forget it.

Mechanism of action

How does ethambutol work?

Blocks arabinosyl transferases, so mycobacteria cannot build the arabinan of arabinogalactan and lipoarabinomannan in their cell wall. It mainly halts growth (bacteriostatic), and given with isoniazid it reduces the emergence of isoniazid resistance. The kidneys clear most of it.

Like other first-line tuberculosis drugs: The four first-line drugs hit Mycobacterium tuberculosis in different ways: isoniazid and ethambutol stop it building its cell wall (mycolic acids and arabinogalactan), rifampicin blocks its RNA polymerase, and pyrazinamide acts on slowly dividing bacilli at acidic pH. Resistance arises fast to any one drug alone, so they are given together: all four for 2 months, then isoniazid and rifampicin for 4.

Uses

What is ethambutol used for?

Pulmonary tuberculosis, with other TB drugs
TB retreatment, with second-line drugs

Side effects

Side effects of ethambutol

Optic neuritis: poor acuity, color blindnessKnown forSerious
Peripheral neuropathy: numbness, tinglingSerious
High uric acid and gout
Liver injurySerious
Nausea, vomiting and abdominal pain
Joint pain
Headache and dizziness
Confusion and hallucinations
Low white cells or plateletsSerious
Hypersensitivity syndrome with rash and organ damageSerious
Lung infiltratesSerious

Known for marks what an exam expects you to know it for.

Watch for

Cautions and what sets it apart

Test visual acuity and color vision before starting and regularly during treatment; stop at the first change.
Avoid in young children and others who cannot report changes in their vision.
Kidney disease: the dose is reduced, as the drug builds up.
Aluminum hydroxide antacids reduce its absorption, so they are kept apart.

Good to know: Often the fourth drug in the first 2 months, guarding against isoniazid resistance; the need for it is reviewed once susceptibility results are known.

Remember it

A way to remember it

RIPE

R: Rifampicin: orange-red urine, sweat and tears
I: Isoniazid: peripheral neuropathy and hepatitis
P: Pyrazinamide: high uric acid and gout
E: Ethambutol: eye damage (optic neuritis)

RIPE names the four first-line drugs, and each letter carries the side effect that drug is best known for.

A mnemonic widely used in teaching

Its class

First-line tuberculosis drugs

Other first-line tuberculosis drugs with a card:

Easily mixed up with fluoroquinolones.

All about first-line tuberculosis drugs

Sources

Checked against

Written for biofideX in plain words, after checking these references:

For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.

Practice it

Antibiotics: DNA, folate and tuberculosis

Ethambutol is one of 11 drugs practiced together here: their classes, how they work, what they treat and what they're known for.

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