An optical fiber biosensor has been presented for monitoring drug delivery inside the body in real time. Over the decades several optical techniques have been realized to monitor drug delivery, however, those are not that much efficient as the one presented here. In this paper, a new micromachined optical fiber biosensor has been demonstrated with both theoretical and numerical models. The detailed fabrication processes have been outlined in a multimode optical fiber which is based on MEMS fabrication. The sensor is designed by following the basic principle of Fabry-Perot interferometer. The optical biosensor presented here could be able to detect and monitor the drug delivery in real time and it could also potentially be used in the area of microfluidics, nano research and other biomedical applications.