A Croatian antibiotic that lingers in tissue for days
Azithromycin was discovered in Croatia in 1980 by the drug firm Pliva. Its trick is staying power: tissue levels can run more than 50 times higher than levels in the blood, and one dose keeps working for several days. It is now among the most prescribed medicines in America and a last oral option against some typhoid.
Approved in 1988, azithromycin is sold as Zithromax in pill form and as Azasite eye drops, and the World Health Organization counts it among its essential medicines. In 2023 American pharmacies filled more than 10 million prescriptions, making it the 64th most common drug. It treats a wide range of bacterial infections, including middle ear infections, strep throat, pneumonia, travellers' diarrhoea and some sexually transmitted infections, and it is used alongside other drugs against malaria.
It belongs to the macrolide antibiotics and stops bacteria from growing by attaching to the large subunit of their ribosomes, which blocks protein production. Its activity is broad but shallow. The staying power comes from chemistry: the drug is trapped inside cells and dissolves readily in fats, and after a single dose its elimination half-life is about 68 hours. Unlike its cousins erythromycin and clarithromycin, which have a 14-membered ring, azithromycin has a 15-membered ring that barely interferes with the liver enzyme CYP3A4. That makes it a safer choice for people on statins.
It has uses beyond killing bacteria. Its anti-inflammatory and immune-calming effects help reduce flare-ups in some severe asthma and in chronic obstructive pulmonary disease. How well it works in asthma seems to depend on the diversity of bacteria in a patient's lungs, and long-term use encourages resistance to macrolides.
Side effects are usually digestive, with diarrhoea in about 5 percent of users, and fewer than 1 percent stop taking it because of them. The rarer risks are serious. In 2013 the American regulator warned that it can disturb the heart's electrical rhythm, potentially fatally, especially in people with existing heart problems. A study of drug-resistant typhoid in South Asia found that resistant strains needed concentrations far beyond what a normal course achieves, and three patients with such infections did not respond.
Source: Azithromycin
