Open label extension studies allow continued prescribing of unapproved drugs after a randomised trial.
Open label extension studies typically follow a double blind randomised placebo controlled trial of a new drug.
At the end of the double blind phase, participants are invited to enroll in an extension study. The study will normally be longer than the randomised trial (two years is not uncommon but they often continue until the drug is licensed). All participants in the extension study are given the study drug, and both they and the investigators know this. The objective is primarily to gather information about safety and tolerability of the new drug in long term, day to day use.
Use of open label studies after phase III trials is relatively common.