Corresponding author. The maximum recommended dose is 300 mg daily.In STIs, the recommended dosages according to the diseases and the recommendations are reported in Table Recommended dosages of doxycycline and other antibiotics for STIs, according to several recommendationsif penicillin allergy: oral doxycycline 100 mg twice daily for 14 dayslate latent syphilis: BPG 2.4 MU/im: 1 injection/week for 3 weeksneurosyphilis: intravenous G penicillin 20 MU/day for 10–15 daysfirst-line therapy option: single dose of BPG 2.4 MU/impenicillin allergy or parenteral treatment refused: oral doxycycline 200 mg daily for 14 days or azithromycin 2 g orally single doseadults primary and secondary syphilis: BPG 2.4 MU/im in a single dosepenicillin allergy: doxycycline 100 mg orally twice daily for 14 daysceftriaxone 250 mg im in a single dose plus azithromycin 1 g orally in a single doseif ceftriaxone is not available: cefixime 400 mg orally in a single dose plus azithromycin 1 g orally in a single doseoral doxycycline 100 mg twice daily for 7 days or single dose of azithromycin 1 glymphogranuloma venereum: oral doxycycline 100 mg twice daily for 21 days or oral erythromycin 500 mg four times daily for 21 daysrecommended regimens: azithromycin 1 g orally in a single dose or doxycycline 100 mg orally twice daily for 7 dayslymphogranuloma venereum recommended regimen: doxycycline 100 mg orally twice daily for 21 daysalternative regimen: erythromycin base 500 mg orally four times daily for 21 daysoral azithromycin 500 mg the first day, 250 mg on the following 4 dayssecond-line treatment: josamycin 1 g twice daily for 14 daysoral azithromycin 500 mg the first day, 250 mg on the following 4 daysif previous treatment failures: moxifloxacin 400 mg daily×7, 10 or 14 daysazithromycin 500 mg on day 1, then 250 mg on days 2 to 5 (oral) or josamycin 500 mg three times daily for 10 daysif macrolide resistance: moxifloxacin 400 mg once daily for 7–10 days (oral)third-line treatment: doxycycline 100 mg twice daily for 14 daysif penicillin allergy: oral doxycycline 100 mg twice daily for 14 dayslate latent syphilis: BPG 2.4 MU/im: 1 injection/week for 3 weeksneurosyphilis: intravenous G penicillin 20 MU/day for 10–15 daysfirst-line therapy option: single dose of BPG 2.4 MU/impenicillin allergy or parenteral treatment refused: oral doxycycline 200 mg daily for 14 days or azithromycin 2 g orally single doseadults primary and secondary syphilis: BPG 2.4 MU/im in a single dosepenicillin allergy: doxycycline 100 mg orally twice daily for 14 daysceftriaxone 250 mg im in a single dose plus azithromycin 1 g orally in a single doseif ceftriaxone is not available: cefixime 400 mg orally in a single dose plus azithromycin 1 g orally in a single doseoral doxycycline 100 mg twice daily for 7 days or single dose of azithromycin 1 glymphogranuloma venereum: oral doxycycline 100 mg twice daily for 21 days or oral erythromycin 500 mg four times daily for 21 daysrecommended regimens: azithromycin 1 g orally in a single dose or doxycycline 100 mg orally twice daily for 7 dayslymphogranuloma venereum recommended regimen: doxycycline 100 mg orally twice daily for 21 daysalternative regimen: erythromycin base 500 mg orally four times daily for 21 daysoral azithromycin 500 mg the first day, 250 mg on the following 4 dayssecond-line treatment: josamycin 1 g twice daily for 14 daysoral azithromycin 500 mg the first day, 250 mg on the following 4 daysif previous treatment failures: moxifloxacin 400 mg daily×7, 10 or 14 daysazithromycin 500 mg on day 1, then 250 mg on days 2 to 5 (oral) or josamycin 500 mg three times daily for 10 daysif macrolide resistance: moxifloxacin 400 mg once daily for 7–10 days (oral)third-line treatment: doxycycline 100 mg twice daily for 14 daysBPG, benzathine penicillin G; im, intramuscular; MU, million units.Recommended dosages of doxycycline and other antibiotics for STIs, according to several recommendationsif penicillin allergy: oral doxycycline 100 mg twice daily for 14 dayslate latent syphilis: BPG 2.4 MU/im: 1 injection/week for 3 weeksneurosyphilis: intravenous G penicillin 20 MU/day for 10–15 daysfirst-line therapy option: single dose of BPG 2.4 MU/impenicillin allergy or parenteral treatment refused: oral doxycycline 200 mg daily for 14 days or azithromycin 2 g orally single doseadults primary and secondary syphilis: BPG 2.4 MU/im in a single dosepenicillin allergy: doxycycline 100 mg orally twice daily for 14 daysceftriaxone 250 mg im in a single dose plus azithromycin 1 g orally in a single doseif ceftriaxone is not available: cefixime 400 mg orally in a single dose plus azithromycin 1 g orally in a single doseoral doxycycline 100 mg twice daily for 7 days or single dose of azithromycin 1 glymphogranuloma venereum: oral doxycycline 100 mg twice daily for 21 days or oral erythromycin 500 mg four times daily for 21 daysrecommended regimens: azithromycin 1 g orally in a single dose or doxycycline 100 mg orally twice daily for 7 dayslymphogranuloma venereum recommended regimen: doxycycline 100 mg orally twice daily for 21 daysalternative regimen: erythromycin base 500 mg orally four times daily for 21 daysoral azithromycin 500 mg the first day, 250 mg on the following 4 dayssecond-line treatment: josamycin 1 g twice daily for 14 daysoral azithromycin 500 mg the first day, 250 mg on the following 4 daysif previous treatment failures: moxifloxacin 400 mg daily×7, 10 or 14 daysazithromycin 500 mg on day 1, then 250 mg on days 2 to 5 (oral) or josamycin 500 mg three times daily for 10 daysif macrolide resistance: moxifloxacin 400 mg once daily for 7–10 days (oral)third-line treatment: doxycycline 100 mg twice daily for 14 daysif penicillin allergy: oral doxycycline 100 mg twice daily for 14 dayslate latent syphilis: BPG 2.4 MU/im: 1 injection/week for 3 weeksneurosyphilis: intravenous G penicillin 20 MU/day for 10–15 daysfirst-line therapy option: single dose of BPG 2.4 MU/impenicillin allergy or parenteral treatment refused: oral doxycycline 200 mg daily for 14 days or azithromycin 2 g orally single doseadults primary and secondary syphilis: BPG 2.4 MU/im in a single dosepenicillin allergy: doxycycline 100 mg orally twice daily for 14 daysceftriaxone 250 mg im in a single dose plus azithromycin 1 g orally in a single doseif ceftriaxone is not available: cefixime 400 mg orally in a single dose plus azithromycin 1 g orally in a single doseoral doxycycline 100 mg twice daily for 7 days or single dose of azithromycin 1 glymphogranuloma venereum: oral doxycycline 100 mg twice daily for 21 days or oral erythromycin 500 mg four times daily for 21 daysrecommended regimens: azithromycin 1 g orally in a single dose or doxycycline 100 mg orally twice daily for 7 dayslymphogranuloma venereum recommended regimen: doxycycline 100 mg orally twice daily for 21 daysalternative regimen: erythromycin base 500 mg orally four times daily for 21 daysoral azithromycin 500 mg the first day, 250 mg on the following 4 dayssecond-line treatment: josamycin 1 g twice daily for 14 daysoral azithromycin 500 mg the first day, 250 mg on the following 4 daysif previous treatment failures: moxifloxacin 400 mg daily×7, 10 or 14 daysazithromycin 500 mg on day 1, then 250 mg on days 2 to 5 (oral) or josamycin 500 mg three times daily for 10 daysif macrolide resistance: moxifloxacin 400 mg once daily for 7–10 days (oral)third-line treatment: doxycycline 100 mg twice daily for 14 daysBPG, benzathine penicillin G; im, intramuscular; MU, million units.STI bacterial agents are innately susceptible to doxycycline; however, the action of tetracyclines on the ribosome and protein synthesis may be affected by 4 mechanisms leading to acquired bacterial resistance: efflux, which is the most common mechanism; drug enzymatic inactivation; target mutation corresponding to mutations in the 16S rRNA gene; and ribosomal protection proteins.Studies investigating antimicrobial resistance of STI pathogens remain generally rare.