Daily doxycycline levels measured to fight STIs

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * Assigned male sex or female sex at birth * In good general health * Not currently taking doxycycline or other tetracycline-derived antibiotics and no plans to initiate during the study * For HIV positive people, on stable antiretroviral therapy with an undetectable viral load and cluster of differentiation 4 (CD4) count\> 300ul/ml * Willing to use condoms consistently for the duration of the study * Able to provide informed consent * No plans for relocation in the next 4 months * Not pregnant and does not plan on getting pregnant for the duration of the study * Willing to undergo peripheral blood, urine, rectal or vaginal secretion collection, and a rectal or vaginal and cervical biopsy procedure * Willing to use study products as directed Exclusion Criteria: * Current or chronic history of liver disease * Continued need for, or use during the 90 days prior to enrollment, of the following medications: * Systemic immunomodulatory agents * Supraphysiologic doses of steroids (short course steroids less than 7 days duration, allowable at the discretion of the investigators) * Chemotherapy or radiation for treatment of malignancy * Experimental medications, vaccines, or biologicals * Intent to use doxycycline or other tetracycline-derived antibiotics during the course of the study, outside of the study procedures * Any other clinical condition or prior therapy that, in the opinion of the investigator, would make the patient unsuitable for the study or unable to comply with the study requirements * Known allergic reaction to study drugs * Significant laboratory abnormalities at baseline visit for rectal biopsies, including but not limited to: * Hemoglobin (Hgb) ≤ 10 g/dL * Partial thromboplastin time (PTT) \> 1.5x upper limit of normal (ULN) or international normalised ratio (INR) \> 1.5x ULN * Platelet count \<100,000