Doxycycline generally stays in your system for about 4 to 6 days after the last dose. This is based on its elimination half-life, which ranges from approximately 16 to 22 hours in healthy adults. The half-life is the time it takes for the concentration of the drug in the bloodstream to reduce by half. Typically, it takes about 5 to 5.5 half-lives for a drug to be completely cleared from the body, which translates to roughly 5 days for doxycycline

. Factors that influence how long doxycycline remains in your system include:

• Dosage and frequency of intake: Higher doses or prolonged use may extend the elimination time.
• Metabolic rate: A slower metabolism can increase the time the drug stays in your body.
• Age and health status: Older age, liver or kidney impairment, and certain medical conditions can slow down clearance.
• Body mass: Larger body mass may prolong drug presence.
• Genetic factors affecting drug metabolism enzymes (such as cytochrome P450)

Doxycycline is metabolized minimally and is excreted mainly via the gut and kidneys. About 40% is eliminated through the kidneys, with the rest via feces

. In summary, while doxycycline's half-life is about 16–22 hours, it takes approximately 4 to 6 days to be fully eliminated from the body, with individual variations depending on health and physiological factors.