Doxycycline monohydrate 100 mg is a widely used antibiotic that begins working soon after the first dose, but the noticeable results depend on the condition being treated. For bacterial infections like respiratory or urinary tract infections, patients may start feeling better within 24 to 48 hours, though full recovery requires completing the prescribed course. In acne treatment, visible improvement may take 2 to 6 weeks, with significant results after several months of consistent use. For malaria prevention, doxycycline must be taken before, during, and after exposure. Always follow medical advice and complete the full course to ensure effectiveness and prevent resistance.