This study is looking at how well the medicine doxycycline works to treat chlamydia, a common infection. Chlamydia is a disease caused by bacteria, and doxycycline is an antibiotic medicine used to treat it. The study will compare two different ways of taking doxycycline: a 3-day treatment and a 7-day treatment. People in the study will take medicine that looks the same to keep the study fair and unbiased. After 28 days, everyone will come back to see if the medicine worked to get rid of the infection.

• The study lasts 28 days, with a return visit for testing.
• Participants must avoid unprotected sex during the study.
• Doxycycline will be given either for 3 days or 7 days.

To join, you need to be at least 16 years old and have untreated chlamydia. You must also be willing to follow the study rules, like not taking other medicines that treat chlamydia. If you are pregnant, planning to become pregnant, or have certain allergies, you might not be able to join. Also, if you have been treated for chlamydia recently or plan to travel during the study time, you may not qualify.