The objective of our study was to measure the effectiveness of Andrographis paniculata SHA-10 extract in reducing the prevalence and intensity of symptoms and signs of common cold as compared with a placebo. A group of 158 adult patients of both sexes completed the randomized double blind study in Valdivia, Chile. The patients were divided in two equal size groups, one of which received Andrographis paniculata dried extract (1200 mg/day) and the other a placebo during a period of 5 days. Evaluations for efficacy were performed by the patient at day 0, 2, and 4 of the treatment; each completed a self-evaluation (VAS) sheet with the following parameters: headache, tiredness, earache, sleeplessness, sore throat, nasal secretion, phlegm, frequency and intensity of cough. In order to quantify the magnitude of the reduction in the prevalence and intensity of the signs and symptoms of common cold, the risk (Odds Ratio = OR) was calculated using a logistic regression model. At day 2 of treatment a significant decrease in the intensity of the symptoms of tiredness (OR = 1.28; 95% CI 1.07-1.53), sleeplessness (OR = 1.71; 95% CI 1.38-2.11), sore throat (OR = 2.3; 95% CI 1.69-3.14) and nasal secretion (OR = 2.51; 95% CI 1.82-3.46) was observed in the Andrographis SHA-10 group as compared with the placebo group. At day 4, a significant decrease in the intensity of all symptoms was observed for the Andrographis paniculata group. The higher OR values were for the following parameters: sore throat (OR = 3.59; 95% CI 2.04-5.35), nasal secretion (OR = 3.27; 95% CI 2.31-4.62) and earache (OR = 3.11; 95% CI 2.01-4.80) for Andrographis paniculata treatment over placebo, respectively. It is concluded that Andrographis paniculata had a high degree of effectiveness in reducing the prevalence and intensity of the symptoms in uncomplicated common cold beginning at day two of treatment. No adverse effects were observed or reported.