An open-label placebo, a pill participants were told plainly contained no active medicine, lowered a hair-based marker of stress hormone in university students preparing for oral exams, and the size of that drop tracked a specific personality trait, according to a randomized controlled trial of 202 students published July 2, 2026 in Comprehensive Psychoneuroendocrinology.
Researchers at MSB Medical School Berlin randomly assigned the students to a four-week open-label placebo course or to no treatment, using an upcoming oral exam as a real-life stress test. Psychological distress was tracked repeatedly through the month; hair cortisol concentration, which reflects average hormone output over roughly the prior month rather than a single moment, was measured before and after in a subsample of 176 students who had usable hair samples. Compared with the no-treatment group, students taking the open-label placebo showed lower reported distress and a larger drop in hair cortisol.
The moderating effect was specific rather than uniform. Students who scored higher on conscientiousness showed the strongest reductions in reported psychological distress, while students who scored higher on neuroticism showed the largest decreases in hair cortisol; extraversion, agreeableness and openness showed no such moderating effect in this trial. The authors concluded the pattern indicates "OLPs may be particularly beneficial for vulnerable individuals," a hypothesis they frame as needing further testing rather than a settled finding.
The trial builds on the same research group's earlier, smaller open-label placebo trial in 134 students, published in Scientific Reports on June 14, 2026, which first linked the honest-placebo intervention to a drop in hair cortisol around exams. The new, larger trial is among the first in this field to pair a physiological outcome with a formal test of which personality traits predict who responds.