BACKGROUND Screening sigmoidoscopy is an underutilized method for detecting early colorectal cancer, and patient discomfort is one reason for poor compliance in the general population. The possible benefit of a well-tolerated, low-cost antispasmodic medication, sublingual hyoscyamine, used before flexible sigmoidoscopy was assessed in a randomized, double-blinded, placebo-controlled trial. METHODS One hundred fifty patients were enrolled and randomized to receive two sublingual hyoscyamine tablets (0.125 mg/tablet) or the placebo 10 minutes before sigmoidoscopy. Patient comfort and the endoscopist's perception of the ease of insertion were measured using a 100 mm visual analog scale. The depth of sigmoidoscope insertion was measured in centimeters, and complications were recorded. RESULTS The median age was 55 years (range 25 to 83 years). There were 100 men (66.7%) and 50 women (33.3%). Approximately half (n = 76, 50.7%) had a prior sigmoidoscopy or colonoscopy. No statistical differences were found between treatment group means for age, gender, pain score, ease of insertion, or depth of insertion. The hyoscyamine group tended to have lower mean pain (32.4 vs. 37.7, p = 0.18) and difficulty (29.9 vs. 33.7, p = 0.31) scores and greater depth of sigmoidoscope insertion (51.3 vs. 47.7, p = 0.07); however, the differences were not statistically significant. The treatment groups differed with a higher percentage of the hyoscyamine group having a previous endoscopy (60.0% vs. 41.3%, p = 0.02); however, no significant differences were detected between mean pain scores as related to treatment when controlling for previous experience with endoscopy (p = 0.31). CONCLUSIONS In this study, hyoscyamine administered in the sublingual route did not significantly improve patient comfort, ease of insertion, or the depth of sigmoidoscope insertion during screening sigmoidoscopy. The search for alternative methods to improve patient comfort during screening endoscopy should continue.