Doxycycline 100mg is an antibiotic that is commonly used to treat bacterial infections. The time it takes for doxycycline to work can vary depending on the type and severity of the infection. In general, you may start to see an improvement in your symptoms within a few days of starting treatment. However, it is important to continue taking the medication as prescribed by your doctor, even if you start to feel better. This will help to ensure that the infection is fully treated and reduce the risk of it coming back. If you have any concerns about how long it is taking for doxycycline to work, or if your symptoms are not improving, you should contact your doctor for further advice.