▁eye ▁reaches ▁the ▁treatment ▁concentration . ▁Con sequently , ▁direct ▁administration ▁through ▁the ▁f ibr ous ▁t unic ▁is ▁common . ▁This ▁is ▁made ▁difficult ▁due ▁to ▁the ▁numerous ▁defense ▁mechan isms ▁in ▁place , ▁such ▁as ▁bl inking , ▁te ar ▁production , ▁and ▁the ▁tight ness ▁of ▁the ▁cor ne al ▁ep ith el ium . ▁Est im ates ▁put ▁te ar ▁turn over ▁rates ▁at ▁ 5 ▁minutes , ▁meaning ▁most ▁conventional ▁dru gs ▁are ▁not ▁retained ▁for ▁long ▁periods ▁of ▁time . ▁Mu co ad hes ives ▁increase ▁ret ention ▁rates , ▁either ▁by ▁en han cing ▁the ▁vis cos ity ▁or ▁bond ing ▁directly ▁to ▁one ▁of ▁the ▁m uc osa e ▁surrounding ▁the ▁eye . ▁ ▁In tra ves ical ▁ ▁In tra ves ical ▁drug ▁administration ▁is ▁the ▁delivery ▁of ▁ph arma ce ut ical s ▁to ▁the ▁ur inary ▁bl ad der ▁through ▁a ▁c ath eter . ▁This ▁route ▁of ▁administration ▁is ▁used ▁for ▁the ▁ther apy ▁of ▁bl ad der ▁cancer ▁and ▁inter st it ial ▁c yst itis . ▁The ▁ret ention ▁of ▁dos age ▁forms ▁in ▁the ▁bl ad der ▁is ▁relatively ▁poor , ▁which ▁is ▁related ▁to ▁the ▁need ▁for ▁a ▁period ical ▁ur ine ▁void ing . ▁Some ▁mu co ad hes ive ▁materials ▁are ▁able ▁to ▁stick ▁to ▁m uc os al ▁l ining ▁in ▁the ▁bl ad der , ▁resist ▁ur ine ▁was h ▁out ▁effects ▁and ▁provide ▁a ▁sust ained ▁drug ▁delivery . ▁ ▁See ▁also