What Is the Difference Between Clarithromycin and Azithromycin?

Clarithromycin and Azithromycin sound fairly similar so it can be easy to get the two of them confused. These two drugs are actually part of the same class of antibiotics called Macrolides. The macrolide antibiotics treat a bacterial infection by stopping the growth and spread of the bacteria rather than directly killing the bacteria. More specifically, macrolides (like Clarithromycin and Azithromycin) bind to the 50S ribosomal subunit in the bacteria to inhibit the synthesis of essential proteins. So then what is the difference between these two antibiotics? In this article, we compare and contrast clarithromycin vs azithromycin to give you a good overview of the differences in key areas including uses, side effects, and cost.

Overview of Macrolide Antibiotics

There are actually several different types of antibiotics, you may have heard of some of them for example Beta-Lactams, or Macrolides. Antibiotics are grouped according to their mechanism of action with the main distinguishing factor being antibiotics that treat infection by directly killing bacteria versus antibiotics that stop the bacteria from growing and spreading. This article focuses on Clarithromycin and Azithromycin which are types of Macrolide antibiotics. As you may have noticed, Macrolides can often be identified by the ‘thromycin’ part of their name. This type of antibiotic is the kind that stops the growth and spread of bacteria; macrolides do this by binding to the 50s ribosomal subunit inside bacteria. What this does is stop the bacteria from making proteins that are needed for growth and survival, thus treating the infection. Macrolide antibiotics like clarithromycin and azithromycin are widely used in clinical care from respiratory infections to sexually transmitted infections. They may also be used in cases where someone has a penicillin allergy.

What Is Clarithromycin?

As mentioned, Clarithromycin is a type of macrolide antibiotic that stops bacteria from being able to make proteins that are necessary for its growth and survival. Clarithromycin can come as a tablet or oral suspension but it can also be administered intravenously. Some cases where Clarithromycin may be used include:

• respiratory infections
• skin infections
• H.pylori

What Is Azithromycin?

Azithromycin is also a type of macrolide antibiotic that binds to the 50s ribosomal subunit to inhibit the spread and growth of bacteria. This antibiotic can take many forms from tablets, capsules, oral suspensions, to intravenous formulations. Some examples of when Azithromycin might be used include:

• sexually transmitted infections, Azithromycin is one of the main Chlamydia medications
• sinusitis
• skin infections

Mechanism of Action: How They Work

As discussed above, Clarithromycin and Azithromycin are both types of macrolide antibiotics. This means that how they work is the same: they treat infections by binding to the 50S ribosomal subunit of bacteria; this disrupts the bacteria’s ability to make proteins necessary for its growth and survival. In addition, Azithromycin can also inhibit proinflammatory chemicals called cytokines which helps to regulate the immune response. 

Spectrum of Activity and Target Infections

With antibiotics, a key consideration for clinical practice is the spectrum of activity and target infections. This means we should check what organism is causing the infection and select an antibiotic that is effective and treating that particular infection. This will take into account factors such as the structure of the bacteria and the mechanism of the drug. For example: 

• Clarithromycin can be used in infections caused by Streptococcus pneumoniae, Haemophilus influenzae, Helicobacter pylori, and Mycoplasma pneumoniae
• Azithromycin can be used in infections caused by Chlamydia trachomatis, Bordetella pertussis, and Legionella

Careful use of antibiotics is important for good stewardship which is of increasing importance in light of global efforts to control antimicrobial resistance. This is when the bacteria evolve to become resistant to antibiotics. Antimicrobial resistance is dangerous because it means more and more antibiotics may not be able to treat even very common and previously simple infections.

Dosage and Duration of Treatment

The dosage and duration of treatment will depend on the condition you are taking the antibiotic for as well as individual factors. Some examples of dosage and duration of treatment include:

• Clarithromycin 200-500mg two times a day for 1-2 weeks. The extended release formulation of Clarithromycin is taken once a day rather than twice.
• Azithromycin is taken as 500mg on the first day, then 250mg once a day for the next four days. However, azithromycin for sexually transmitted infections is typically administered as a single 1g dose. 

As you can see, there can be considerable variation in the treatment plan for Clarithromycin and Azithromycin despite both of them being the same type of antibiotic. Make sure you follow your doctor’s instructions and finish your course of antibiotics as recommended. 

Side Effects and Safety Profile

Despite being highly effective and widely used, as with all medication macrolide antibiotics like Clarithromycin and Azithromycin do come with a small risk of side effects. Some more common ones include:

• gastrointestinal disturbances e.g nausea, vomiting, diarrhoea
  • risk of nausea and vomiting is fairly similar between Clarithromycin and Azithromycin
  • risk of diarrhoea is higher with Azithromycin 
• headaches
  • risk is slightly higher with Clarithromycin
• changes to taste e.g metallic taste
  • risk is higher with Clarithromycin
• sleep disturbances
  • risk is higher with Clarithromycin

Some more serious side effects watch out for include:

• changes to the rhythm of your heart (specifically QT prolongation)
• liver damage
• anaphylaxis or severe allergic reaction

These are usually rare but if you notice symptoms of any of these serious side effects, seek medical attention promptly.

Drug Interactions and Precautions

If you are taking any other medication (including herbal medication), please inform your healthcare provider or consult a pharmacist before starting Clarithromycin/Azithromycin. This safety precaution is important because certain medications may interact with the antibiotic and cause side effects or increase/decrease effectiveness of treatment. Clarithromycin in particular interacts with hundreds of different medications. Some examples of drug interactions include:

• Clarithromycin and statins, warfarin, benzodiazepines
• Azithromycin and medication that also affects heart rhythm (QT prolonging)

Efficacy and Resistance Considerations

Both Clarithromycin and Azithromycin are highly effective antibiotics in treating infections from common pathogens (particularly in the skin and respiratory system). Azithromycin has a longer half-life and penetrates the tissue better than Clarithromycin. This antibiotic may be preferred when the infection is intracellular or in cases where compliance to treatment may be more difficult. Regardless, the choice of antibiotic and the patient’s completion of the course of treatment is very important to efforts to combat antimicrobial resistance. Careless or overuse of antibiotics may mean that more bacteria evolve to become resistant to our current arsenal of antibiotics which can be dangerous as treatments become less effective. 

Cost and Availability

The cost and availability of antibiotics can vary greatly depending on whether you are getting it privately from online providers like Meds for Less or on the NHS. On the NHS, a course of Clarithromycin will cost the standard NHS prescription fee of around £9.90 while private prescriptions (e.g from online providers) for a course of Clarithromycin will often cost upwards of £30. Azithromycin on the NHS will also cost £9.90 while from a private provider it may cost around £10-20. 

Which One Is Better for Your Condition?

Deciding between Azithromycin and Clarithromycin is a clinical decision. At the consultation with your doctor, explain your symptoms, past medical history, and any regular medication so they can make an informed assessment of which antibiotic is most suited to your condition. Some examples of macrolide antibiotics and conditions they are commonly used in include:

• Azithromycin for:
  • Chlamydia 
  • Chronic bronchitis
• Clarithromycin for:
  • H.pylori infection
  • Lyme disease

Summary: Key Differences at a Glance

Hopefully you now have a better understanding of the difference between azithromycin and clarithromycin. Although Clarithromycin and Azithromycin are both macrolide antibiotics, there are still considerable differences between the two and the decision on which one to use is best reserved for an informed discussion with a medical professional. The table below notes some key differences to outline a comparison of the two antibiotics: