Took The Wrong Dose Of ZPak

Hello and thanks for reaching out!

A Z-Pak, which contains the antibiotic azithromycin, consists of six (6) 250mg tablets for a 5-day course of therapy. It is prescribed as:

• Take two tablets (500mg) by mouth as one dose on the first day
• Take one tablet (250mg) daily thereafter for 4 more days

Based on your question, it does indeed appear that the wrong dose of your Z-Pak was taken. You took 500mg on day one and on day two (by accident), and plan on taking one daily thereafter, which will leave you one day short.

While there isn't any data or studies that I am aware of that have evaluated the effectiveness of this different dosing regimen, in all likelihood, the drug will still be effective simply based on how the drug works, and how it is metabolized in our system. I talk about this in the next sections.

3-Day Vs. 5-Day Regimen

I want to point out something very important for your situation. I'd reach out to your doctor as soon as possible because an alternative dosage regimen for azithromycin is to take 500mg once daily for 3 days. Studies show this dosage regimen is essentially as effective as the 5-day 'Z-Pak' dosing for most indications, so could be an option for you since you have already taken 500mg for two days in a row and you have enough tablets on hand to finish this regimen.

In fact, azithromycin is commercially available in a 'Tri-Pack' formulation, that contains three (3) 500mg tablets intended for a 3-day course of daily dosing. Below is an image of both a 'Z-Pak' and a 'Tri-Pak':

Why A Z-Pak Is Dosed The Way It Is

Azithromycin is an antibiotic with a very long half-life, around 68 hours, and therefore, lasts a long time in your system. The first dose of 500mg from your Z-Pak is a 'loading-dose', which allows the drug to reach an effective concentration in your system. The 250mg daily doses thereafter are to maintain that effective concentration.

Since azithromycin has a long half-life, it is still working to fight your infection long after you take your last dose, at least for 3 to 5 days after.

In fact, azithromycin lasts so long in your system, that, depending on what you are treating, it can sometimes be taken as a single 1,000mg dose. Alternative dosage regimens exist as well (including the 3-day regimen I talked about earlier).

Wrong Dosing Guidance

Going back to your question, in all likelihood, the drug will still be effective as you essentially took two loading doses and have two more tablets for two days. Since the drug lasts so long in your system, there is a little leeway for this incorrect dose, and the drug will be at a concentration that is effective in eradicating bacteria.

Having said this though, I would still reach out to your doctor since the medication wasn't taken as prescribed, and they know your complete medical situation as well as the dosing that will be most effective for you.

They may tell you to continue with taking one tablet daily, which likely will still be very effective for treating your infection. Alternatively, they may have you take 500mg on day three, which is an FDA-approved alternative dosage regimen for some conditions and nearly as effective as a 5-day regimen. Since I don't know what you are treating, it is also possible they recommend a different drug or regimen altogether.

Final Words

I hope this answer helped! Please reach out again in the future if anything comes up!