Purpose: To estimate sufficiently important difference (SID) for common cold. SID is the amount of benefit sufficient to justify treatment costs and risks.

Methods: Hypothetical benefits of reduction in duration and severity were assessed. We presented treatment scenarios based on best evidence regarding vitamin C, the herbal echinacea, zinc lozenges, and pleconaril, an unapproved antiviral. Costs and risks were held constant. Hypothetical benefits were varied until the cold-sufferer indicated the treatment was desirable.

Results: In Cohort A, we assessed duration SID with 162 telephone and 149 in-person interviewees. For the vitamin, mean SID was 26.1 hours (95% CI=23.2...29.3), with 31% accepting without any duration benefit, and 5% rejecting even with a 4-day reduction-in-duration benefit. For the herbal, SID was 36.8 hours (33.4...40.2), with 23% accepting, and 9% rejecting. For the lozenge, SID was 64.8 hours (61.0...67.9) with 9% accepting and 24% rejecting. For the antiviral, SID was 82.6 hours (78.7...86.7), with 6% accepting and 48% rejecting. An overall SID of 52.6 hours (50.6...54.6) was determined by averaging across the four scenarios.

In Cohort B, we assessed severity SID with 144 telephone and 49 in-person interviewees. For the vitamin, mean SID was 24.0% (20.8%...27.2%), with 13 % saying they would take it regardless of severity benefit, and 1% saying they would not, even if severity benefit exceeded 75%. For the herbal, SID was 31.8% (28.3%...35.3%), with 6% accepting and 7% rejecting. For the lozenge, SID was 45.0% (41.5%...48.5%), with 2% accepting and 4% rejecting. For the antiviral, SID was 56.4% (52.2%...60.6%), with 2% accepting and 36% rejecting. Overall SID was estimated at 39.3% (36.9%...41.7%) by averaging across scenarios.

Differences in SID among treatment scenarios were not due to chance. Age, sex, and severity of illness did not significantly influence response patterns. Qualitative interviews suggested that side effects, treatment type (tablet vs. lozenge vs. herbal extract), monetary costs, and opportunity costs may be important influences.

Conclusions: On average, people want the duration of their colds to be reduced by 26-to-65 hours in order to justify costs and risks of popular cold treatments. A prescription antiviral would require more benefit (83 hours). Alternatively, common cold treatments would need to reduce global severity by 24%-to-45% to justify costs and risks. A prescription antiviral would require a 56% severity-reduction-benefit.