Clarithromycin
Is It Right for You?
A complete guide to Clarithromycin — what it treats, how it works, dosages, side effects, and when a clinician may prescribe it following an online consultation.
What Is Clarithromycin?
Clarithromycin is a broad-spectrum macrolide antibiotic used to treat a wide range of bacterial infections affecting the respiratory tract, skin, and stomach. It is one of the most widely used antibiotics in UK primary care and is particularly valued for its effectiveness against both typical and atypical bacterial pathogens, its excellent tissue penetration, and its suitability as an alternative antibiotic for patients who are allergic to penicillin.
Clarithromycin was developed in the 1980s as a chemically modified derivative of erythromycin, designed to overcome some of erythromycin's significant limitations – including its poor acid stability, short duration of action, and frequent and often severe gastrointestinal side effects. Clarithromycin offers improved acid stability, a longer half-life allowing twice-daily dosing, better tolerability, and enhanced activity against key respiratory pathogens including Haemophilus influenzae.
It is available in standard tablet, modified-release tablet, and oral suspension forms, and is suitable for adults and children. Clarithromycin is available generically and under brand names including Klaricid.
Clarithromycin is a prescription-only medication in the UK. A licensed clinician must assess your symptoms and medical history before it can be prescribed – which can now be done quickly and conveniently through a telehealth consultation.
What Conditions Is Clarithromycin Used For?
Clarithromycin is prescribed for a variety of bacterial infections across multiple body systems:
community-acquired pneumonia including atypical pneumonia caused by Mycoplasma pneumoniae and Chlamydophila pneumoniae; acute exacerbations of chronic bronchitis; acute bronchitis where an antibiotic is warranted.
streptococcal pharyngitis and tonsillitis, particularly in penicillin-allergic patients.
acute bacterial sinusitis, particularly in penicillin-allergic patients or where first-line antibiotics have failed.
acute otitis media, particularly where first-line treatment is unsuitable or has failed.
mild to moderate infections caused by susceptible organisms including Staphylococcus aureus and Streptococcus pyogenes, particularly in penicillin-allergic patients.
a key component of standard triple therapy regimens used to eradicate H. pylori from the stomach lining in patients with peptic ulcers or gastritis; typically combined with a proton pump inhibitor and amoxicillin or metronidazole.
used in combination regimens for Mycobacterium avium complex (MAC) infections in immunocompromised patients.
first-line treatment for early Lyme disease in adults and children over 12.
dental abscesses and severe dental infections caused by oral bacteria including anaerobes.
Clarithromycin is an antibiotic and will not treat the common cold, influenza, COVID-19, or any other viral illness. Using antibiotics unnecessarily contributes to antibiotic resistance. A clinician will assess whether your symptoms are likely bacterial or viral before prescribing.
How Does Clarithromycin Work?
Clarithromycin belongs to the macrolide class of antibiotics and exerts its antibacterial effect by inhibiting bacterial protein synthesis. It penetrates bacterial cells and binds to the 50S ribosomal subunit – the component of the bacterial ribosome responsible for assembling amino acids into proteins. By binding to this subunit, clarithromycin blocks the translocation step of protein synthesis – the process by which the ribosome moves along the messenger RNA strand to read genetic instructions and build new protein chains.
Without the ability to produce essential proteins, bacteria cannot grow, replicate, maintain their structure, or carry out vital metabolic functions. At standard therapeutic doses, clarithromycin is primarily bacteriostatic – it inhibits bacterial growth and replication, allowing the body's immune system to clear the infection. At higher concentrations achieved in tissues due to its excellent distribution, it may be bactericidal against some organisms.
One of clarithromycin's major clinical advantages is its broad spectrum of activity, covering:
- Gram-positive organisms including Staphylococcus aureus and Streptococcus pneumoniae
- Gram-negative organisms including Haemophilus influenzae and Moraxella catarrhalis
- Atypical intracellular organisms including Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophila
- Helicobacter pylori
- Mycobacterial species
Clarithromycin also has a unique additional mechanism: it is metabolised in the body to an active metabolite called 14-hydroxyclarithromycin, which has its own independent antibacterial activity and works synergistically with the parent compound – particularly against Haemophilus influenzae – enhancing overall efficacy.
Dosages & Administration
The correct dose depends on the type and severity of infection, age, weight, and kidney function. Always follow your clinician's instructions. The table below is for general reference only.
Administration Tips
Clarithromycin standard tablets can be taken with or without food. Food does not significantly affect absorption and may help reduce gastrointestinal side effects such as nausea.
Modified-release tablets (Klaricid XL) should be taken with food and swallowed whole – do not crush, chew, or break them, as this destroys the modified-release mechanism and alters drug delivery.
Take doses at evenly spaced intervals – twice-daily dosing should ideally be taken approximately 12 hours apart to maintain consistent blood levels throughout the day and night.
Oral suspension should be shaken well before each use. Store at room temperature (not in the refrigerator) and discard after 14 days.
Always complete the full prescribed course even if symptoms improve rapidly. For H. pylori eradication therapy in particular, completing the full 7-day triple therapy course is essential for successful eradication.
Side Effects of Clarithromycin
Most side effects are mild and resolve once treatment is completed. Gastrointestinal effects are the most commonly reported. Serious side effects are less common but require prompt medical attention.
- Nausea
- Diarrhoea
- Vomiting
- Abdominal discomfort or pain
- Indigestion or heartburn
- Headache
- Altered or metallic taste in the mouth (dysgeusia) – a distinctive and commonly reported effect
- Oral or vaginal thrush
- Cardiac arrhythmias (QT prolongation) – clarithromycin can prolong the QT interval and trigger serious, potentially fatal heart arrhythmias, particularly in patients with pre-existing cardiac conditions or electrolyte abnormalities
- Severe allergic reaction (anaphylaxis) – sudden onset hives, facial or throat swelling, difficulty breathing, collapse
- Cholestatic hepatitis and liver failure – jaundice, dark urine, pale stools, severe fatigue; rare but serious; can occur during or after treatment
- Stevens-Johnson Syndrome / Toxic Epidermal Necrolysis – rare but severe blistering skin reactions
- C. difficile-associated diarrhoea – persistent watery or bloody diarrhoea, abdominal cramping, fever
- Psychiatric effects – anxiety, confusion, hallucinations, and depersonalisation have been reported, particularly in elderly patients; discontinue and seek medical advice if these occur
- Hearing loss or tinnitus – rare; more likely with high doses or prolonged treatment
Clarithromycin has a significant and well-documented association with QT interval prolongation. This can lead to a serious cardiac arrhythmia known as Torsades de Pointes. Risk is highest in patients with existing cardiac conditions, electrolyte abnormalities (low potassium or magnesium), or those taking other QT-prolonging medications. Always disclose your full cardiac and medication history before being prescribed clarithromycin.
Drug Interactions
Clarithromycin is a potent inhibitor of the liver enzyme CYP3A4, which is responsible for metabolising a large number of commonly prescribed medications. This makes clarithromycin one of the antibiotics with the most clinically significant drug interaction profile. Always disclose all prescription drugs, over-the-counter medicines, vitamins, and supplements to your clinician or pharmacist before starting clarithromycin.
Clarithromycin significantly
inhibits the metabolism of statins via CYP3A4, causing a dramatic increase in statin blood
levels. This raises the risk of serious muscle damage (rhabdomyolysis). Simvastatin and
lovastatin should be temporarily stopped during clarithromycin treatment. Other statins should
be used with caution and at reduced doses.
Combining clarithromycin with other QT-prolonging medications substantially
increases the risk of dangerous cardiac arrhythmias. This combination should generally be
avoided or only used under close medical supervision with cardiac monitoring.
Clarithromycin markedly increases blood
levels of ergot alkaloids by inhibiting their metabolism, potentially causing severe ergotism with
vasoconstriction and limb ischaemia. Concurrent use is contraindicated.
Clarithromycin inhibits the elimination of colchicine, leading to potentially
toxic blood levels. Serious and fatal colchicine toxicity has been reported. This combination
should be avoided, particularly in patients with renal or hepatic impairment.
Clarithromycin significantly increases blood
levels of these immunosuppressants, raising the risk of nephrotoxicity and other serious
toxicities. Close monitoring of drug levels and renal function is essential if concurrent use cannot
be avoided.
Clarithromycin enhances the anticoagulant effect of warfarin,
significantly increasing bleeding risk. INR must be monitored closely during and shortly after
clarithromycin treatment in patients taking warfarin.
Clarithromycin increases digoxin absorption and reduces its renal elimination, raising plasma levels and the risk of digoxin toxicity. Digoxin levels and signs of toxicity (nausea, visual disturbances, arrhythmias) should be monitored.
Clarithromycin inhibits the metabolism of some benzodiazepines, leading to prolonged and enhanced sedative effects. Avoid concurrent use of oral midazolam or triazolam with clarithromycin.
As with other antibiotics, there is a theoretical risk of reduced contraceptive efficacy. Use additional contraceptive precautions during treatment and for 7 days after completing the course.
Clarithromycin may increase theophylline blood levels. Monitoring of theophylline levels and signs of toxicity (palpitations, seizures, nausea) is advisable in patients on theophylline therapy.
Important Warnings
Clarithromycin carries a well-established risk of QT interval prolongation and has been associated with cases of serious and potentially fatal ventricular arrhythmias. This risk is particularly significant in elderly patients, those with pre-existing cardiac disease, electrolyte abnormalities, or those taking other QT-prolonging drugs. Some guidelines recommend avoiding clarithromycin in patients with known cardiovascular disease where alternative antibiotics are available. Seek immediate medical attention if you develop palpitations, irregular heartbeat, fainting, or dizziness during treatment
Clarithromycin inhibits the CYP3A4 enzyme system responsible for metabolising many commonly prescribed medications. This makes the potential for serious drug interactions higher with clarithromycin than with most other antibiotics. It is essential that your clinician and pharmacist are aware of every medication you are taking before clarithromycin is prescribed. Do not start clarithromycin without a full medication review
Clarithromycin has been associated with hepatotoxicity including cholestatic hepatitis and, rarely, liver failure. Risk is higher with prolonged use and in patients with pre-existing liver disease. Clarithromycin should be used with caution or avoided in patients with significant hepatic impairment. Contact your clinician if you develop jaundice, dark urine, pale stools, or severe fatigue during or after treatment.
Neuropsychiatric side effects including anxiety, confusion, hallucinations, nightmares, and depersonalisation have been reported with clarithromycin. These are more common in elderly patients. If you or those around you notice unusual changes in mood, thought, or behaviour during treatment, stop clarithromycin and contact a clinician promptly.
Like all antibiotics, clarithromycin can disrupt gut flora and allow Clostridioides difficile to overgrow. If you develop persistent, watery, or bloody diarrhoea during or after treatment, contact a clinician promptly and do not self-treat with anti-diarrhoeal medications.
Clarithromycin is generally not recommended during pregnancy, particularly in the first trimester, as some studies have suggested a possible association with adverse pregnancy outcomes. Alternative antibiotics are usually preferred. Clarithromycin passes into breast milk and is not recommended during breastfeeding. Always inform your clinician if you are pregnant or breastfeeding before starting any antibiotic.
Speak to a Clinician About Treatment
Getting advice no longer means sitting in a waiting room. Through The GP Service, you can consult with a licensed clinician in minutes — from home, on your lunch break, or wherever works for you. If treatment is clinically appropriate, your clinician can issue a prescription during the consultation. A consultation does not guarantee a prescription.
The treatment you need, when you need it.
Clarithromycin FAQs
Yes. A licensed clinician can assess your symptoms via a telehealth consultation and prescribe clarithromycin if it is clinically appropriate. Prescriptions are typically sent to your preferred pharmacy the same day, often within minutes of your consultation.
Clarithromycin is well absorbed and begins working within a few hours of the first dose. Most patients notice an improvement in symptoms within 48 to 72 hours. If symptoms are worsening significantly after 48 hours or not improving after 72 hours, contact your clinician for reassessment.
An altered or metallic taste – known medically as dysgeusia – is one of the most commonly reported and distinctive side effects of clarithromycin. It is caused by the drug being excreted into saliva and is harmless. It resolves completely once treatment is finished. Drinking water, chewing sugar-free gum, or eating can temporarily reduce the sensation.
Yes. Clarithromycin is a macrolide antibiotic with no structural relationship to penicillin and is a commonly used and effective alternative for patients who are allergic to penicillin. It is suitable for treating respiratory infections, throat infections, skin infections, and dental infections in this group.
Both are macrolide antibiotics with similar mechanisms of action. The key differences are: clarithromycin requires twice-daily dosing for 7–14 days while azithromycin typically requires once-daily dosing for just 3–5 days due to its longer half-life; clarithromycin is the preferred macrolide for H. pylori eradication; clarithromycin has a significantly greater number of drug interactions due to its potent CYP3A4 inhibition; and azithromycin is generally better tolerated gastrointestinally. Your clinician will choose the most appropriate option for your specific infection and circumstances
This depends on which statin you take. Clarithromycin significantly increases blood levels of some statins – particularly simvastatin and lovastatin – to potentially dangerous levels, raising the risk of serious muscle damage. If you take a statin, inform your clinician before starting clarithromycin. You may be advised to temporarily stop your statin during treatment, or an alternative antibiotic may be prescribed.
There is no known dangerous direct interaction between alcohol and clarithromycin. However, alcohol can impair the immune system, slow recovery from infection, and may worsen nausea, which is already a common side effect of clarithromycin. It is advisable to avoid or limit alcohol during treatment.
Take the missed dose as soon as you remember. If it is almost time for your next scheduled dose, skip the missed one and continue as normal. Do not take a double dose to compensate. Try to maintain an approximately 12-hour interval between doses where possible.
Yes. Clarithromycin is licensed for use in children and is available in oral suspension form. Paediatric doses are calculated based on body weight. It is commonly used in children who are allergic to penicillin.
Yes. Clarithromycin is a key component of the standard triple therapy used to eradicate Helicobacter pylori – the bacterial cause of most peptic ulcers and chronic gastritis. It is typically combined with a proton pump inhibitor (such as omeprazole or lansoprazole) and either amoxicillin or metronidazole for a 7-day course. Successful eradication significantly reduces the risk of ulcer recurrence.