Azithromycin

Azithromycin belongs to the macrolide family. It binds to the 23S portion of the bacterium’s 50S ribosomal subunit, the machinery that builds its proteins. By attaching there, it blocks the passage of aminoacyl-tRNA and the elongation of the peptide chain, which halts bacterial protein synthesis. Its action is mainly bacteriostatic: it prevents bacteria from multiplying, and becomes bactericidal at high doses against certain germs.

This target is specific to bacteria. Azithromycin has no effect on viruses: it treats neither the common cold, nor influenza, nor other viral infections. That is why a doctor reserves it for confirmed or strongly suspected bacterial infections.

Azithromycin is used to treat a wide range of bacterial infections. It is prescribed for community-acquired pneumonia, middle ear infection, pharyngitis, skin infections and exacerbations of chronic obstructive pulmonary disease. It is also used in several sexually transmitted infections: chlamydia, gonococcus and Mycoplasma genitalium.

It also covers more specific situations, such as Mycobacterium avium infections in patients with AIDS. The choice of the drug and the duration depend on the targeted germ and the site of the infection, which the doctor determines on a case-by-case basis.

Azithromycin has a striking pharmacological feature: its mean terminal half-life is around 68 hours. This slow elimination allows short courses and a single daily dose, with the drug’s effect continuing long after each dose. Its oral bioavailability is about 38 percent, and it is eliminated mainly through the bile.

It concentrates strongly inside cells, particularly phagocytes, with an intracellular to extracellular concentration ratio greater than 30 after one hour. It thus reaches higher concentrations in tissues than in plasma, closest to the sites of infection. This tissue accumulation partly explains the effectiveness of such brief treatments.

Azithromycin is taken once a day, with or without food, at a regular time. For the tablet, taking it with a meal increases the maximum concentration (Cmax) by 23 percent, without changing the total exposure (AUC). The usual adult regimen ranges from 250 to 500 mg per day for 3 to 5 days, or a single 1 g dose for a chlamydia infection.

Regularity of intake determines the treatment’s effectiveness. You must complete the prescribed course, even if symptoms improve before the end, and not stop it on your own initiative. Antacids containing aluminum or magnesium should be taken separately from azithromycin, because they can interfere with its absorption.

The most frequent adverse effects are digestive. In multiple-dose clinical trials in adults, diarrhea or loose stools are seen in 4 to 5 percent of people, nausea in 3 percent, abdominal pain in 2 to 3 percent, and vomiting in 1 percent or less. These disturbances are partly explained by the activation of motilin receptors, which stimulates intestinal motility.

Serious effects remain rare. There are reports of liver injury (hepatotoxicity), severe skin reactions such as Stevens-Johnson syndrome, and major allergic reactions (anaphylaxis). These situations call for stopping the treatment and seeking care without delay.

Like other macrolides, azithromycin can prolong the heart’s QT interval, with rare cases of torsades de pointes, a serious rhythm disturbance. A lowered and uncorrected potassium or magnesium level is a pro-arrhythmic condition that raises this risk. Several interactions call for caution: the combination with pimozide is contraindicated, colchicine (a P-glycoprotein substrate) calls for vigilance, and monitoring is reinforced with nelfinavir.

In patients on warfarin, the prothrombin time (INR) is monitored, because the combination can alter its balance. Monitoring relies mainly on clinical observation: the courses are short and stopping for an adverse effect remains rare. Attention is paid to signs of liver injury, and the INR is checked in people on oral anticoagulants.