Within the eye, it is difficult to achieve therapeutic concentrations through systemic administration. Often, other parts of the body will reach toxic levels of the medication before the eye reaches the treatment concentration. Consequently, direct administration through the fibrous tunic is common. This is made difficult due to the numerous defense mechanisms in place, such as blinking, tear production, and the tightness of the corneal epithelium. Estimates put tear turnover rates at 5 minutes, meaning most conventional drugs are not retained for long periods of time. Mucoadhesives increase retention rates, either by enhancing the viscosity or bonding directly to one of the mucosae surrounding the eye.