Wikipedia wrote:

Azithromycin is commonly administered in tablet or oral suspension (a one-dose version was made available in 2005). Tablets come in 250 mg and 500 mg doses. Oral suspension comes in 100 mg/teaspoon and 200 mg/teaspoon strengths. The 250 mg tablets are often dispensed in packages of six and commonly referred to as a "Z-Pak," whereas the 500 mg tablets are commonly available commercially in a pack of three tablets, or "Tri-Pak," intended as a three-day treatment. A common dose of oral azithromycin therapy consists of a "double dose" of medication on the first day of treatment and subsequent treatment for four or five additional days. With the "Z-Pak," this means two 250 mg tablets (a total of 500 mg) on the first day and one 250 mg tablet once daily for the next four days.