Clarithromycin is a macrolide antibiotic used to treat a wide range of bacterial infections, including pneumonia, bronchitis, sinus infections, ear infections, skin infections, and strep throat. It also plays a role in treating stomach ulcers caused by H. pylori bacteria and in preventing a specific lung infection in people with HIV. Most courses last 7 to 14 days, with adults typically taking 250 to 500 mg twice daily.
How Clarithromycin Works
Clarithromycin belongs to the macrolide class of antibiotics. It stops bacteria from growing by attaching to a specific part of the bacterial ribosome, the cellular machinery bacteria use to build proteins. Without the ability to produce proteins, the bacteria can’t multiply or repair themselves, which gives your immune system the upper hand to clear the infection. This mechanism makes it effective against a broad spectrum of bacteria, particularly those that infect the respiratory tract, skin, and digestive system.
Respiratory and Ear Infections
The most common reason doctors prescribe clarithromycin is for infections of the airways, sinuses, and ears. It treats community-acquired pneumonia (the kind you pick up outside of a hospital), acute bronchitis, and bacterial sinusitis. For ear infections and strep throat, it serves as a useful alternative for people who are allergic to penicillin. A typical course for these infections runs 7 to 14 days at 250 to 500 mg taken every 12 hours, or 1,000 mg once daily if you’re taking the extended-release tablet.
Children six months and older can also take clarithromycin, usually in liquid suspension form. Their dose is calculated by body weight, generally 7.5 mg per kilogram every 12 hours for about 10 days.
H. pylori and Stomach Ulcers
Clarithromycin has long been a cornerstone of treatment for Helicobacter pylori, the bacterium responsible for most stomach and duodenal ulcers. In this role, it’s never used alone. Instead, it’s combined with a proton pump inhibitor (a stomach acid reducer) and one or two other antibiotics, typically amoxicillin. Treatment lasts 10 to 14 days.
However, the landscape here is shifting. Resistance to clarithromycin among H. pylori strains has risen significantly in many parts of the world. The American College of Gastroenterology now recommends against using clarithromycin-based triple therapy unless lab testing has confirmed the bacteria are still sensitive to it. If your doctor suspects resistance, they’ll likely choose a different combination, such as a bismuth-based four-drug regimen or newer alternatives.
MAC Prevention in People With HIV
Mycobacterium avium complex (MAC) is a type of bacterial infection that primarily affects the lungs and can spread throughout the body in people with severely weakened immune systems, particularly those living with HIV. Clarithromycin is used both to treat active MAC infections and to prevent them from developing in the first place. The preventive and treatment dose is typically 500 mg twice daily, and it’s usually combined with other antimicrobial medications for active infection. Children 20 months and older can also receive clarithromycin for MAC, dosed by weight up to a maximum of 500 mg twice daily.
Less Common Uses
Beyond its primary indications, clarithromycin is sometimes prescribed for infections that fall outside its standard label. These include Lyme disease, Legionnaires’ disease (a serious form of pneumonia caused by Legionella bacteria), whooping cough, cat scratch disease, and cryptosporidiosis (a parasitic infection that causes watery diarrhea). It’s also occasionally given before dental procedures to people at high risk of heart valve infections, as a preventive measure against bacteria entering the bloodstream.
Common Side Effects
The most frequently reported side effect is an altered or metallic taste in the mouth, affecting roughly 16% of people who take it. Nausea is nearly as common, occurring in about 12% of users. Diarrhea affects somewhere between 1% and 10%. These side effects are generally mild and resolve once the course is finished. Taking the medication with food can help reduce stomach upset.
Heart Rhythm Risks
Clarithromycin carries a more serious, though uncommon, risk: it can affect the heart’s electrical rhythm by prolonging what’s called the QT interval, which controls the timing between heartbeats. A large Danish cohort study found that current use of clarithromycin was associated with an increased risk of cardiac-related death compared to penicillin, a pattern also seen with other macrolide antibiotics like erythromycin and azithromycin. This risk is most relevant for people who already have heart conditions, are taking other medications that affect heart rhythm, or have low potassium or magnesium levels. If you have a known heart condition, your doctor will weigh this risk before prescribing clarithromycin.
Drug Interactions to Know About
Clarithromycin is processed by the liver through a pathway that also handles many other common medications. This means it can increase blood levels of certain drugs, potentially making their side effects worse. Medications that interact significantly include some cholesterol-lowering statins, blood thinners, seizure medications, and drugs used for heart rhythm problems. If you’re taking other prescription medications, your pharmacist will check for interactions before filling the prescription. Grapefruit juice can also interfere with clarithromycin’s metabolism, so it’s worth mentioning to your pharmacist if that’s part of your routine.