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Macrolide antibiotic Pregnancy: UK SPC §4.6, verbatim. PREGNANCY: 'The safety of clarithromycin for use during pregnancy has not been established. Based on variable results obtained from animal studies and experience in humans, the possibility of adverse effects on embryofoetal development cannot be excluded. Some observational studies evaluating exposure to clarithromycin during the first and second trimester have reported an increased risk of miscarriage compared to no antibiotic use or other antibiotic use during the same period. The available epidemiological studies on the risk of major congenital malformations with use of macrolides including clarithromycin during pregnancy provide conflicting results. Therefore, use during pregnancy is not advised without carefully weighing the benefits against risks.' §4.4 adds: 'The physician should not prescribe clarithromycin to pregnant women without carefully weighing the benefits against risk, particularly during the first three months of pregnancy.' BREAST-FEEDING: 'The safety of clarithromycin for use during breast feeding of infants has not been established. Clarithromycin is excreted into human breast milk in small amounts. It has been estimated that an exclusively breastfed infant would receive about 1.7% of the maternal weight-adjusted dose of clarithromycin.' FERTILITY: 'In the rat, fertility studies have not shown any evidence of harmful effects.' US §8.1 is stronger: 'Based on findings from animal studies, clarithromycin tablets are not recommended for use in pregnant women except in clinical circumstances where no alternative therapy is appropriate' — animal studies produced cardiovascular anomalies in rats and cleft palate in mice at clinically relevant doses.

Clarithromycin 500mg (Atypical CAP)

Brand names: Klaricid (250 mg and 500 mg tablets), Klaricid XL (500 mg modified release)

This is clarithromycin used as a macrolide antibacterial for respiratory tract infection, including cover for atypical pathogens in community-acquired pneumonia. It may be used alone for atypical cover or added to a beta-lactam in more severe pneumonia.

Adult dose

Dose: 250 mg twice daily, increased to 500 mg twice daily in severe infections (respiratory tract infection; adults and children older than 12 years)
Route: Oral — immediate-release film-coated tablets. UK SPC §4.2: 'Clarithromycin may be given without regard to meals as food does not affect the extent of bioavailability.' US §2.1: 'Clarithromycin tablets may be given with or without food.'
Frequency: Twice daily. UK SPC §4.2: 'The usual duration of treatment is 6 to 14 days.' US label Table 1 gives community-acquired pneumonia as 250 mg every 12 hours for 7-14 days (7 days where the pathogen is H. influenzae) and acute bacterial exacerbation of chronic bronchitis as 250 to 500 mg every 12 hours for 7-14 days.
Max: No maximum is stated for respiratory tract infection in the fetched labelling. The highest respiratory dosage stated anywhere in it is 500 mg twice daily — UK SPC §4.2, verbatim: 'The usual dose is 250 mg twice daily although this may be increased to 500mg twice daily in severe infections.' Higher or more frequent regimens in the fetched text (H. pylori eradication at 500 mg every 8 hours, and mycobacterial infection) belong to different indications and are not ceilings for this one.
VERBATIM UK SPC §4.2 (Patients with respiratory tract/skin and soft tissue infections) — 'Adults: The usual dose is 250 mg twice daily although this may be increased to 500mg twice daily in severe infections. The usual duration of treatment is 6 to 14 days. Children older than 12 years: As for adults. Children younger than 12 years: Use of Clarithromycin film-coated tablets is not recommended for children younger than 12 years. Clinical trials have been conducted using clarithromycin paediatric suspension in children 6 months to 12 years of age. Therefore, children under 12 years of age should use clarithromycin paediatric suspension.' ELDERLY (§4.2): 'As for adults.' — but US §8.5 adds 'Consider dosage adjustment in elderly patients with severe renal impairment' and 'Elderly patients may be more susceptible to development of torsades de pointes arrhythmias than younger patients.' FORMULATION SCOPE: this page describes 500 mg BD for 5-7 days, a Klaricid XL 500 mg once-daily modified-release option and an intravenous option; only the immediate-release oral tablet is in this bundle, and its stated duration is 6 to 14 days (UK) / 7-14 days for community-acquired pneumonia (US Table 1) — the 5-7 day course on the page is not sourced here. DIFFERENT INDICATIONS, quoted only so they are not mistaken for the respiratory dose — H. pylori eradication in duodenal ulcer (adults), UK §4.2: 'Triple Therapy Clarithromycin (500mg) twice daily and lansoprazole 30mg twice daily should be given with amoxicillin 1000mg twice daily'; 'Triple Therapy Clarithromycin (500mg) twice daily and lansoprazole 30mg twice daily should be given with metronidazole 400mg twice daily'; 'Triple Therapy Clarithromycin (500mg) twice daily and omeprazole 40mg daily should be given with amoxycillin 1000mg twice daily or metronidazole 400mg twice daily'; 'Triple Therapy Clarithromycin (500mg) twice daily and omeprazole 20mg daily should be given with amoxycillin 1000mg twice daily.' Mycobacterial infection (US §2.5): 'For treatment and prophylaxis of mycobacterial infections in adults, the recommended dose of clarithromycin tablets is 500 mg every 12 hours.' DOSE ADJUSTMENT FOR INTERACTIONS (US §2.7): 'Decrease the dose of clarithromycin tablets by 50 % when co-administered with atazanavir.' Clarithromycin inhibits CYP3A: US §7 advises using it with caution with other CYP3A substrates, 'especially if the CYP3A substrate has a narrow safety margin (e.g., carbamazepine)', adjusting dosage where appropriate and monitoring serum concentrations.

Paediatric dose

Dose: 15 mg/kg/day
Route: Oral
Frequency: Total daily dose divided every 12 hours, for 10 days
Max: Maximum — the adult dose: US label §2.4 caps the paediatric daily dosage 'up to the adult dose'. For respiratory tract infection that adult dose is 250 mg twice daily, increased to 500 mg twice daily in severe infections (UK SPC §4.2). No separate numerical paediatric ceiling is stated for this indication.
US FDA label §2.4, verbatim and in full: 'The recommended daily dosage is 15 mg/kg/day divided every 12 hours for 10 days (up to the adult dose).' This is a DAILY TOTAL to be split into two doses — not a per-dose figure — hence the unit mg/kg/day. US §8.4: safety and effectiveness are established in patients 6 months and older for pharyngitis/tonsillitis, community-acquired pneumonia, acute maxillary sinusitis, acute otitis media and uncomplicated skin and skin structure infections; 'Safety and effectiveness of clarithromycin in pediatric patients under 6 months of age have not been established.' UK SPC §4.2: the film-coated tablets 'are not recommended for children younger than 12 years… children under 12 years of age should use clarithromycin paediatric suspension'; children older than 12 years are dosed 'As for adults'. NO suspension strength appears anywhere in this bundle (the US label's §3 covers 250 mg and 500 mg tablets only), so concentration is null and no administration volume can be calculated here — take the strength from the product being dispensed. A DIFFERENT indication, mycobacterial infection (US §2.5), uses 'the recommended dose is 7.5 mg/kg every 12 hours up to 500 mg every 12 hours' — that is a per-dose figure for mycobacterial disease and must not be applied to pneumonia, and it is not the source of the value above. Confirm paediatric dosing against a children's formulary and local antimicrobial guidance before prescribing.

Dose adjustments

Renal

UK SPC §4.2, verbatim: 'In patients with renal impairment with creatinine clearance less than 30 mL/min, the dosage of clarithromycin should be reduced by one-half, i.e. 250 mg once daily, or 250 mg twice daily in more severe infections. Treatment should not be continued beyond 14 days in these patients.' US label §2.6 (Table 2) is more granular: severe renal impairment (creatinine clearance <30 mL/min) — reduce the dosage by 50%; moderate renal impairment (creatinine clearance 30 to 60 mL/min) taking concomitant atazanavir or ritonavir-containing regimens — reduce by 50%; severe renal impairment taking concomitant atazanavir or ritonavir-containing regimens — reduce by 75%. UK §4.4 adds: 'Caution should also be exercised when administering clarithromycin to patients with moderate to severe renal impairment.'

Hepatic

No hepatic dose reduction is stated in the fetched labelling. UK SPC §4.4, verbatim: 'Clarithromycin is principally metabolised by the liver. Therefore, caution should be exercised in administering this antibiotic to patients with impaired hepatic function.' UK §4.3 makes one combination absolute: 'Clarithromycin should not be used in patients who suffer from severe hepatic failure in combination with renal impairment.' US §4.3 adds a further contraindication: 'Clarithromycin tablets are contraindicated in patients with a history of cholestatic jaundice or hepatic dysfunction associated with prior use of clarithromycin.' §4.4 warns that hepatic dysfunction — increased liver enzymes, hepatocellular and/or cholestatic hepatitis with or without jaundice — 'may be severe and is usually reversible', with cases of fatal hepatic failure reported; stop treatment if signs of hepatic disease develop.

Dose independently re-sourced from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — clinician-reviewed. Always confirm against the product SmPC and your local formulary before prescribing.

Paediatric weight-based calculator

US FDA label §2.4, verbatim and in full: 'The recommended daily dosage is 15 mg/kg/day divided every 12 hours for 10 days (up to the adult dose).' This is a DAILY TOTAL to be split into two doses — not a per-dose figure — hence the unit mg/kg/day. US §8.4: safety and effectiveness are established in patients 6 months and older for pharyngitis/tonsillitis, community-acquired pneumonia, acute maxillary sinusitis, acute otitis media and uncomplicated skin and skin structure infections; 'Safety and effectiveness of clarithromycin in pediatric patients under 6 months of age have not been established.' UK SPC §4.2: the film-coated tablets 'are not recommended for children younger than 12 years… children under 12 years of age should use clarithromycin paediatric suspension'; children older than 12 years are dosed 'As for adults'. NO suspension strength appears anywhere in this bundle (the US label's §3 covers 250 mg and 500 mg tablets only), so concentration is null and no administration volume can be calculated here — take the strength from the product being dispensed. A DIFFERENT indication, mycobacterial infection (US §2.5), uses 'the recommended dose is 7.5 mg/kg every 12 hours up to 500 mg every 12 hours' — that is a per-dose figure for mycobacterial disease and must not be applied to pneumonia, and it is not the source of the value above. Confirm paediatric dosing against a children's formulary and local antimicrobial guidance before prescribing.

Verify in a children's formulary

Contraindications

  • Known hypersensitivity to clarithromycin, to other macrolide antibiotic drugs or to any of the excipients (UK SPC §4.3; US §4.1 names clarithromycin, erythromycin or any macrolide antibacterial)
  • History of QT prolongation (congenital or documented acquired) or ventricular cardiac arrhythmia, including torsades de pointes (UK SPC §4.3)
  • Electrolyte disturbances — hypokalaemia or hypomagnesaemia, due to the risk of prolongation of the QT interval (UK SPC §4.3)
  • Concomitant ergot alkaloids, e.g. ergotamine or dihydroergotamine — risk of ergot toxicity (UK SPC §4.3)
  • Concomitant oral midazolam (UK SPC §4.3)
  • Concomitant lomitapide (UK SPC §4.3; also US §4.5)
  • Concomitant astemizole, cisapride, domperidone, pimozide or terfenadine — risk of QT prolongation and cardiac arrhythmias including ventricular tachycardia, ventricular fibrillation and torsades de pointes (UK SPC §4.3; US §4.2 lists cisapride and pimozide)
  • Concomitant ticagrelor, ivabradine or ranolazine (UK SPC §4.3)
  • Concomitant HMG-CoA reductase inhibitors extensively metabolised by CYP3A4 — lovastatin or simvastatin — due to the increased risk of myopathy including rhabdomyolysis (UK SPC §4.3; US §4.5)
  • Concomitant colchicine — 'As with other strong CYP3A4 inhibitors, Clarithromycin should not be used in patients taking colchicine' (UK SPC §4.3); the US label restricts this to colchicine in renal or hepatic impairment (§4.4)
  • Severe hepatic failure in combination with renal impairment (UK SPC §4.3)
  • History of cholestatic jaundice or hepatic dysfunction associated with prior use of clarithromycin (US §4.3 — not listed in the fetched UK §4.3)
  • Concomitant lurasidone (US §4.7 — not listed in the fetched UK §4.3)

Side effects

  • Most frequent and common in both adults and children (UK §4.8a): abdominal pain, diarrhoea, nausea, vomiting and taste perversion — 'usually mild in intensity and … consistent with the known safety profile of macrolide antibiotics'. US §6.1 lists the same set as abdominal pain, diarrhea, nausea, vomiting and dysgeusia
  • Gastrointestinal, common (UK §4.8b): diarrhoea, vomiting, dyspepsia, nausea, abdominal pain; uncommon: oesophagitis, gastro-oesophageal reflux disease, gastritis, proctalgia, stomatitis, glossitis, abdominal distension, constipation, dry mouth (the table is truncated at this point)
  • Nervous system, common: dysgeusia, headache; uncommon: loss of consciousness, dyskinesia, dizziness, somnolence, tremor; not known: convulsion, ageusia, parosmia, anosmia, paraesthesia
  • Psychiatric, common: insomnia; uncommon: anxiety, nervousness; not known: psychotic disorder, confusional state, depersonalisation, depression, disorientation, hallucination, abnormal dreams, mania
  • Cardiac, uncommon: cardiac arrest, atrial fibrillation, electrocardiogram QT prolonged, extrasystoles, palpitations; not known: torsades de pointes, ventricular tachycardia, ventricular fibrillation
  • Infections, common/uncommon: cellulitis, candidiasis, gastroenteritis, vaginal infection; not known: pseudomembranous colitis, erysipelas. UK §4.4: Clostridioides difficile-associated diarrhoea has been reported and 'may range in severity from mild diarrhoea to fatal colitis', occurring up to two months after antibacterial use
  • Hepatobiliary (UK §4.4): hepatic dysfunction including increased liver enzymes and hepatocellular and/or cholestatic hepatitis, with or without jaundice — 'may be severe and is usually reversible'; cases of fatal hepatic failure have been reported
  • Blood and lymphatic, uncommon: leukopenia, neutropenia, thrombocythemia, eosinophilia; not known: agranulocytosis, thrombocytopenia
  • Immune system, uncommon: anaphylactoid reaction, hypersensitivity; not known: anaphylactic reaction, angioedema
  • Ear and labyrinth, uncommon: vertigo, hearing impaired, tinnitus; not known: deafness
  • US §5.5 warning: risk of all-cause mortality one year or more after the end of treatment in patients with coronary artery disease — 'Balance this potential risk with the treatment benefits when prescribing'
  • US §5.8 warning: exacerbation of myasthenia gravis has been reported
  • This side-effect list may be incomplete — check the full SPC.

Monitoring

  • Clinical response and duration — UK §4.2: usual treatment 6 to 14 days; in creatinine clearance less than 30 mL/min 'treatment should not be continued beyond 14 days in these patients'
  • Hepatic: UK §4.4 — 'Patients should be advised to stop treatment and contact their doctor if signs and symptoms of hepatic disease develop, such as anorexia, jaundice, dark urine, pruritus, or tender abdomen'
  • Diarrhoea: UK §4.4 — consider Clostridioides difficile-associated diarrhoea in any patient with diarrhoea after antibiotic use (it has been reported over two months later), consider discontinuing clarithromycin 'regardless of the indication', perform microbial testing and avoid drugs inhibiting peristalsis
  • ECG/QT: US Table 8 — 'Electrocardiograms should be monitored for QTc prolongation during coadministration of clarithromycin with' disopyramide or quinidine; US §5.2 advises avoiding clarithromycin in patients with known QT prolongation, ventricular arrhythmia, hypokalaemia/hypomagnesaemia, significant bradycardia or Class IA or III antiarrhythmics
  • Drug interactions: US §7 — 'Adjust dosage when appropriate and monitor serum concentrations of drugs primarily metabolized by CYP3A closely in patients concurrently receiving clarithromycin'
  • Renal function in the elderly: US §8.5 — 'Consider dosage adjustment in elderly patients with severe renal impairment'; colchicine toxicity has been reported especially in elderly patients and those with renal insufficiency, with deaths reported
  • No routine laboratory monitoring schedule is specified in the fetched sections

Clinical monograph

How it works

Clarithromycin inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit, giving activity against typical respiratory bacteria and atypicals such as Mycoplasma, Chlamydophila, and Legionella. Its action is largely bacteriostatic.

Prescribing in practice

  • It prolongs the QT interval and is a potent CYP3A4 inhibitor, so review for QT-prolonging and interacting drugs (notably statins, which should be paused, and certain anticoagulants) before prescribing.
  • Avoid in known macrolide hypersensitivity and use with caution in significant cardiac or hepatic disease and electrolyte disturbance.
  • Adjust the dose in significant renal impairment in line with current prescribing references.

Monitoring

Monitor clinical response, liver function on prolonged courses, and ECG or electrolytes where there is additional QT risk.

Counselling the patient

  • Complete the full prescribed course even once you feel better.
  • Report palpitations, fainting, jaundice, or severe diarrhoea.
  • Mention all other medicines, as clarithromycin interacts with many drugs.

Evidence & guidelines

NICE community-acquired pneumonia guidance positions a macrolide such as clarithromycin for atypical cover, alone or combined with a beta-lactam by severity.

Reference: NICE NG120 Pneumonia; NICE CKS LRTI; BTS CAP Guidelines 2009 (updated); 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.

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