Frequent instillation of drops, (e.g. hourly hypromellose), may cause a sensitivity reaction to the preservative in the drops.
Hypromellose eye drops (with preservative) are the traditional choice of treatment for tear deficiency. It is the cheapest and the least viscous.
Acetylcysteine 5% with hypromellose 0.35% (Ilube®) drops are useful as a mucolytic agent for dry eye with a major mucus component.