- Fifteen years ago landmark studies were performed with pilocarpine and timolol. In retrospect, the principal limitations of these studies were that they empirically studied only one duration (5 minutes) of EC or NLO, and did not include (because they hadn't yet been invented) what are now the most commonly used eye drops to lower IOP, the prostaglandins. The limitation of the 5 minute duration is that it may be impractical for many patients and decrease adherence due to its inconvenience.