When taking a Z-Pack (azithromycin), also known as a Z-PAK, for infection, one of the most important aspects of successful therapy is completing the entire dosage, which is a total of 1500 mg (6 tablets of 250 mg).

There are two commercial types of Z-Packs, a 5 day course and a 3 day course. The 5 day course is the most commonly prescribed with the directions of two tablets as one dose on the first day, then one tablet once daily for 4 days. The 3 day course is one tablet daily for 3 days. They both have a total therapy dosage of 1500 mg.

When studied, both courses (3 day vs. 5 day) have similar effectiveness and reach similar concentrations in the body. Dosage regimens that vary slightly from the usual exist too, and they all tend to work just as well as one another. This is most likely due to how azithromycin is metabolized and eliminated from the body. It has a relatively long half life (time it takes to eliminate 50% of the drug) when compared to other antibiotics at 68 hours. This long half-life is due to tissue uptake in the body and the drug releasing slowly over time.

When you miss a dose with a Z-Pack, as you have, it is very important to complete the entire course of therapy (6 tablets of 250 mg).  Therefore, it is important to continue on the dosing schedule (once daily) and finish the entire pack. There should not be a significant decrease in effectiveness, if any. 

It is not uncommon to not feel better after completing therapy. As described above, azithromycin lasts a long time in the body and continues to work for days after your Z-Pack is finished.

If your symptoms don't seem to be improving after about a week or two, contact your doctor.