Time course of anti-nicotine-specific antibody formation in mice immunized with various nicotine vaccines. Mice (n = 8 per group) were immunized by subcutaneous injection on days 0, 14, and 28. In the blank group, mice were injected with phosphate-buffered saline as the negative control. Antibody titers were compared among groups using one-way ANOVA followed by Tukey's HSD test. Data are expressed as means ± SD. Significantly different antibody titers: *p < 0.05, **p < 0.01, ***p < 0.001. Antibody titers were significantly different compared to that of the previous study time point: # < 0.05, ### < 0.001.