How blind is double-blind? A study of fish oil versus placebo.

The distinctive aftertaste associated with fish oil preparations used in clinical trials of omega-3 fatty acids may weaken the double-blind. This double-blind pilot study was designed to examine whether normal subjects could correctly 'guess' if they were receiving capsules containing concentrated fish oil or capsules of pure olive oil. The informed consent was designed to give subjects ambiguous expectations about what oil they might be receiving to examine whether this would influence their guess. Despite a marked difference in taste experience, there was no significant difference in correct guesses between the two groups. The results suggest that altering subjects' expectations could further improve the validity of the double-blind.