In dental procedures performed under intravenous sedation in patients with intellectual disabilities, procedures are sometimes interrupted by the cough reflex, which may be triggered by intraoral use of water and/or increased secretion stimulating the pharyngeal/laryngeal mucosa, or by those irritating the tracheal mucosa directly through anesthetics-induced impairment of the laryngeal closure reflex. We investigated relationships between frequency of coughing episodes and intraoral use of water, water remaining in the oropharyngeal space, and mean infusion rate of propofol during dental treatments performed under intravenous sedation with midazolam and propofol. Twenty-one intellectually disabled patients were enrolled. After induction, a 14 Fr. suction catheter was inserted nasally, which was fixed where oropharyngeal suction could be done most effectively. Patients were divided into three groups according to the amount of intraoral use of water, amount of oropharyngeal suction and mean infusion rate of propofol. The amount of oropharyngeal suction significantly correlated with intraoral use of water. Frequency of coughing episodes significantly correlated with amount of oropharyngeal suction per minute. However, coughing episodes correlated neither with intraoral use of water nor with infusion rate of propofol. These findings suggested that accumulation of water in the oropharynx increased vulnerability to the cough reflex in dental treatments performed under intravenous sedation.