The Z-Pak (azithromycin 250mg tablets) is one of the most commonly prescribed antibiotics and is used a variety of infections, including bronchitis, sinusitis and ear infections. The dosing of the Z-Pak is as follows:
- Two tablets (500 mg) on first day of therapy, followed by one tablet (250 mg) once daily for 4 days. The total duration of therapy is 5 days.
It is very important that the two tablets (500 mg) used for the first dose are taken at the same time. This 500 mg dose acts as a "loading dose" and allows the medication to reach high enough concentrations in your body to kill the infection causing bacteria.
Azithromycin has a unique characteristic in that it has an extremely long-half life (around 68 hours), meaning that each doses lasts for days in the body. The 500 mg day one loading dose essentially is used to produce sufficiently high azithromycin levels in your body, which is then maintained with the 250 mg daily doses for the next four days. If you do not take a loading dose, the concentrations of azithromycin may not reach high enough levels to be effective.
Per the prescribing information for azithromycin:
"With a regimen of 500 mg on Day 1 and 250 mg/day on Days 2-5, Cmin [miniumum concentrations] and Cmax [maximum concentrations] remained essentially unchanged from Day 2 through Day 5 of therapy. However, without a loading dose, azithromycin Cmin concentrations required 5 to 7 days to reach steady-state."
In other words, if you don't take a loading dose, you will experience lower concentrations of the drug, which may not be effective, and therapy will most likely need to be lengthened to eliminate your infection.
If you accidentally took one tablet (250 mg) instead of two (500 mg), you should talk to your doctor as they may alter your therapy to ensure effectiveness.