Diphenhydramine has local anaesthetic and antimicrobial activity and may be used to prevent intravenous propofol injection pain. We studied the effect of adding diphenhydramine to propofol emulsions for preventing bacterial growth.

Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli and Candida albicans cultures were inoculated into the following solutions: 1% propofol, 0.05% diphenhydramine + 1% propofol, 0.1% diphenhydramine + 1% propofol, 0.2% diphenhydramine + 1% propofol, 0.3% diphenhydramine + 1% propofol, 1% diphenhydramine and 0.1% lidocaine + 1% propofol. A 100-μL of inoculum suspension adjusted for each of the micro-organisms was added separately to each tube and left at 20°C. A 10-μL aliquot of each mixture was inoculated onto blood agar medium at 5 and 24 h. These plates were incubated at 35°C for 24 h. Each plated medium was read, and the number of colony-forming units were counted and recorded (n = 2). Analysis of variance (ANOVA) with a post hoc Tukey HSD test and paired t-tests were used for data analysis. P < 0.05 was considered as significant.

Diphenhydramine inhibited bacterial growth in propofol solutions in a dose-dependent manner. It was more effective than 0.1% lidocaine at similar concentrations in preventing bacterial growth for all organisms.

Diphenhydramine had a significant inhibitory effect on bacterial growth in propofol.