After enrolment into the study and randomization to the study group (V0) and baseline examinations (V1), patients will receive 1 drop of Lacrimera at the study site and will be instructed to apply one drop of C-NAC into both eyes before bedtime for the following 5 consecutive days and return for a follow-up visit after 7±1 days (V2). Patients will complete a diary describing symptoms during the day, use of Lacrimera and potential side effects until the next visit. If the NEI grading is ≤1 OR the OSDI score <20 at the next visit, treatment with Lacrimera will be discontinued and one further physical follow-up appointment will be arranged, which will take place 28±2 days after the last physical visit.

The study days are conducted in the same way as for the study group. Patients of the control group will receive preservative-free, Hyaluronic-acid containing eyedrops (Hylo-Vision® sine) for 4 times a day as control.

A new preservative-free formulation of eye drops consists of a novel biopolymer, chitosan-N-acetylcysteine (C-NAC; Lacrimera®, Croma-Pharma GmbH, Leobendorf, Austria), which electrostatically binds to the mucine layer of the tear film forming a glycocalyx-like structure.

Duration of treatment in days until objectively confirmed recovery (NEI (National Eye Institute) grading ≤1 OR OSDI (Ocular Surface Disease Index) score <20, either 7±1, 14±1, 21±1 or 28±1 days) in the study group compared to the control group.

Change in BUT (Break Up Time) at different visits in both groups. Fluorescein Break Up time is measured after instillation of 100 μl fluorescein into the inferior fornix. Using a stopwatch time after the last blink until the first break up of the tear film is recorded. Three consecutive measurements are performed, and the average is calculated.

Non-invasive first and averaged BUT is measured three times and averaged afterwards using the Dry eye module build into the corneal tomography device (Sirius, CSO, Italy).