Erythromycin

ERYTHROCIN

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Pronunciation: er-ith-roe-MYE-sin

Erythromycin ballicule: receptor binding, kinetics and half-life diagram — Mechanism of action: Erythromycin ballicule

Erythromycin E-MYCIN mascot mnemonic cartoon — Visual mnemonic: Erythromycin mascot
**“(Eryth) throw your mice in here”**
macrolide ring
erythro = red

What it isMacrolide inhibiting the 50S ribosomal subunit; atypical infection, and used off-label as a prokinetic.

Why this oneIts motilin agonism makes it useful for gastroparesis, which no other antibiotic offers.

What limits itStrong CYP3A4 inhibitor and QT prolonger, so interactions are constant. GI upset limits the dose; tachyphylaxis develops in prokinetic use.

Erythromycin molecular structure (skeletal formula) — Molecular structure: Erythromycin

Erythromycin prescription history

Animated chart of estimated U.S. prescriptions per year, Antibacterials (systemic) (1938–2026). Erythromycin enters in 1952 (estimated).

FDA-approved for

bacterial infections (1952)
pertussis (1952)

Principal riskA strong CYP3A4 inhibitor; QT prolongation

Classes and tags

Clinical profile

Therapeutic safety burden1 / 4

Overdose danger1 / 4

Weight-gain liability0 / 4

Sedation liability0 / 4

QT / Torsades3 / 4

Direct muscarinic antagonism0 / 4

Embryofetal risk1 / 4

Legacy pregnancy categoryC

Defining liabilityAllergy, gastrointestinal effects and agent-specific renal, hepatic, hematologic or interaction risks.

FDA label

Read the full label on DailyMed ↗

NO BOXED WARNINGS

FDA dosing	Population	Start	Target	Max
Susceptible infections (usual)	Adults	250 mg QID (equally spaced) or 500 mg q12h	Increase per severity of infection	4 g/day (BID dosing not recommended >1 g/day)
Streptococcal URTI (tonsillitis/pharyngitis); prevention of initial rheumatic fever attacks (penicillin-allergic)	Adults	Usual therapeutic dose	≥10 days	4 g/day
Long-term prophylaxis of recurrent rheumatic fever (allergic to penicillin and sulfonamides)	Adults	250 mg BID	250 mg BID	250 mg BID
Urogenital C. trachomatis infection during pregnancy	Pregnant women	500 mg QID on empty stomach ×≥7 days	If not tolerated: 500 mg q12h or 250 mg QID ×≥14 days	500 mg QID
Uncomplicated urethral/endocervical/rectal C. trachomatis; nongonococcal urethritis (U. urealyticum) — when tetracycline contraindicated/not tolerated	Adults	500 mg QID ×≥7 days	500 mg QID	500 mg QID
Primary syphilis (penicillin-allergic)	Adults	30–40 g total in divided doses over 10–15 days		40 g total course
Acute PID caused by N. gonorrhoeae	Adult females (penicillin-sensitive)	500 mg erythromycin lactobionate IV q6h ×3 days	then 500 mg base PO q12h ×7 days	Undefined
Intestinal amebiasis	Adults	500 mg q12h or 250 mg q6h ×10–14 days		Undefined
Legionnaires' disease	Adults	1–4 g/day in divided doses (optimal dose not established)		4 g/day
Susceptible infections (usual)	Children	30–50 mg/kg/day in equally divided doses	May double for more severe infections	4 g/day
Intestinal amebiasis	Children	30–50 mg/kg/day divided ×10–14 days		Undefined
Pertussis	Children	40–50 mg/kg/day divided ×5–14 days (optimal dose not established)		Undefined
Conjunctivitis of the newborn (C. trachomatis)	Newborns	Oral suspension 50 mg/kg/day in 4 divided doses ×≥2 weeks		Undefined
Pneumonia of infancy (C. trachomatis)	Infants	Oral suspension 50 mg/kg/day in 4 divided doses ×≥3 weeks		Undefined

Renal Undefined

Hepatic Use with caution in impaired hepatic function (principally excreted by the liver); no dose adjustment stated

Instructions

May be taken without regard to meals; optimal levels when fasting (at least 1/2 hour, preferably 2 hours before meals)
Streptococcal URTI: treat for at least 10 days

Available strengths

tab 250, 500 mg (base/stearate)
tab 400 mg (EES)

Contraindications

Concomitant terfenadine, astemizole, cisapride, pimozide, ergotamine, or dihydroergotamine
Concomitant CYP3A4-metabolized statins (lovastatin, simvastatin) — myopathy/rhabdomyolysis risk

Cautions

Hepatotoxicity
QT prolongation / torsades de pointes
Syphilis in pregnancy — may not prevent congenital syphilis
C. difficile-associated diarrhea
Serious CYP3A4 drug interactions (colchicine toxicity, statin rhabdomyolysis, CCB hypotension)

Adverse reactions

Nausea
Vomiting
Abdominal pain
Diarrhea
Anorexia
Hepatic dysfunction/abnormal LFTs
Allergic reactions (urticaria to anaphylaxis)
Reversible hearing loss (renal insufficiency, high doses)

Pregnancy No teratogenicity in animal studies; observational human studies reported cardiovascular malformations after early-pregnancy exposure

Lactation Excreted in human milk; use with caution in nursing women

Cost

Erythromycin (ERYTHROCIN)

Strength		#/mo	CostPlus
250 mg	tab	120 QID	not offered
500 mg	tab	120 QID	not offered
400 mg	tab	120 QID	not offered

NADAC is a voluntary CMS survey of what retail community pharmacies pay for the drugs they stock. Erythromycin is dispensed outside retail pharmacies, so it rarely appears on surveyed invoices.

Generic entry1982

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Memory aids: interaction avatars

CYP3A4 inhibitor (moderate)

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Self-check

Which receptor explains erythromycin’s prokinetic effect?
Motilin receptor.
What limits sustained erythromycin prokinetic benefit?
Tachyphylaxis.