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Azithromycin (Z-Pack / Zithromax)

Antibioticprescriptionoral · inferred

Azalide macrolide antibiotic with exceptionally long tissue half-life due to extensive intracellular uptake and slow release from tissues. Serum levels are low but tissue concentrations (especially lungs, tonsils, prostate) remain therapeutic for days after the last dose. Unique dosing: 500 mg day 1 then 250 mg days 2-5. Inhibits bacterial 50S ribosomal subunit. CYP3A4 interactions minimal compared to erythromycin/clarithromycin.

Projected serum levels — 250 mg, once daily

Azithromycin (Z-Pack / Zithromax)
Azithromycin (Z-Pack / Zithromax) modeled serum levels, 250 mg once daily over 23 days Population-based pharmacokinetic estimate. Steady state reached after approximately 7 days. 0 125 250 375 500 Day 0 Day 6 Day 12 Day 17 Day 23 Time on a regular schedule ≈ steady state · day 7
Azithromycin (Z-Pack / Zithromax) modeled serum levels with a loading dose of 750 mg, then 250 mg once daily The first dose is larger so levels approach steady state faster. 0 125 250 375 500 Day 0 Day 6 Day 12 Day 17 Day 23 Time on a regular schedule

Maintenance schedule: 250 mg once daily (oral).

Loading schedule: 750 mg on day 1, then 250 mg once daily — reaching therapeutic levels sooner.

Modeled steady state after ~7 days: peak ≈ 408 mg, trough ≈ 324 mg body load. Population-based estimate over 23 days for a 250 mg dose at a 70 kg reference body mass — the interactive app scales curves to your doses, timing, and body mass.

Key facts

Category
Antibiotic
Route modeled
Oral
Model confidence
inferred
Half-life
2.8 days
Common dose
250 mg
Suggested maximum
500 mg/day
Reference dose range
125–500 mg (single dose)
Suggested cadence
once daily
Validated against
500 mg oral — Cmax 0.41 mg/L at 2.5 h

Documented interactions

  • contraindicated
    Carbamazepine (Tegretol) + Azithromycin (Z-Pack / Zithromax) CYP induction

    Carbamazepine (Tegretol) inducer of CYP3A4 predicted to change Azithromycin (Z-Pack / Zithromax) AUC by ~0.20x

  • contraindicated
    Clarithromycin (Biaxin) + Azithromycin (Z-Pack / Zithromax) CYP inhibition

    Clarithromycin (Biaxin) mechanism_based of CYP3A4 predicted to change Azithromycin (Z-Pack / Zithromax) AUC by ~10.00x

  • contraindicated
    Dasatinib (Sprycel) + Azithromycin (Z-Pack / Zithromax) CYP inhibition

    Dasatinib (Sprycel) time_dependent_inhibitor of CYP3A4 predicted to change Azithromycin (Z-Pack / Zithromax) AUC by ~5.00x

  • contraindicated
    Diltiazem + Azithromycin (Z-Pack / Zithromax) CYP inhibition

    Diltiazem mechanism_based of CYP3A4 predicted to change Azithromycin (Z-Pack / Zithromax) AUC by ~10.00x

  • contraindicated
    Efavirenz (Sustiva) + Azithromycin (Z-Pack / Zithromax) CYP induction

    Efavirenz (Sustiva) inducer of CYP3A4 predicted to change Azithromycin (Z-Pack / Zithromax) AUC by ~0.12x

  • contraindicated
    Grapefruit (whole fruit) + Azithromycin (Z-Pack / Zithromax) CYP inhibition

    Grapefruit (whole fruit) mechanism_based of CYP3A4 predicted to change Azithromycin (Z-Pack / Zithromax) AUC by ~10.00x

Serum checks 109 modeled interaction pairings for azithromycin (z-pack / zithromax) across your whole stack — absorption conflicts, enzyme inhibition, and nutrient depletion included.

Research behind this entry

  1. Azithromycin: mechanisms of action and their relevance for clinical applications Parnham MJ et al. · Pharmacology & Therapeutics, 2014 DOI

    Comprehensive review covering azithromycin's unique tissue pharmacokinetics (tissue:serum ratios >100:1), anti-inflammatory properties, and mechanism of intracellular accumulation in phagocytes.

  2. Pharmacokinetics of azithromycin and the concentration of azithromycin in polymorphonuclear leukocytes and monocytes Foulds G et al. · Journal of Antimicrobial Chemotherapy, 1990 DOI

    Pivotal PK study establishing azithromycin terminal half-life of 68 hours, extensive tissue distribution, and demonstrating drug concentrations in WBCs remain above MIC for most pathogens for 5+ days after dosing.

2 published studies referenced in the app, each with a plain-language summary.