An open-label study dispenses with the concealment of treatment assignments that characterizes blinded trials, allowing all parties to know which intervention each participant receives. This transparency can be either a deliberate design choice or a practical necessity. Some studies are designed as open-label from the outset, while others may include open-label phases following completion of a blinded treatment period, allowing participants who received placebo to access active treatment.

Open-label designs are employed in various contexts where blinding is impractical, unnecessary, or unethical. Studies of surgical interventions, medical devices, or treatments with distinctive side effects may be impossible to blind convincingly. Early-phase studies exploring safety and tolerability may use open-label designs when precise efficacy assessment is not the primary objective. Long-term extension studies often operate as open-label to provide continued access to promising treatments while gathering additional safety data. Studies comparing treatments that require fundamentally different administration procedures may necessitate open-label conduct.

The absence of blinding in open-label studies introduces potential for bias that must be acknowledged and addressed. Participants who know they are receiving active treatment may report more favorable outcomes due to expectations, while those aware of receiving placebo may report worse outcomes or discontinue prematurely. Investigators may unconsciously assess or record outcomes differently based on treatment knowledge. These biases are particularly concerning for subjective endpoints. Open-label studies should therefore employ objective outcome measures when possible, use blinded endpoint adjudication committees, and interpret results with appropriate caution regarding the potential influence of unblinded assessment.