Excess nicotinamide increases plasma serotonin and histamine levels (2015)

Nicotinamide supplementation increases plasma serotonin and histamine, because it depletes the available methyl groups in your body. Those methyl groups are involved in a lot of processes. Two of which are the breakdown of serotonin and histamine. They also noticed a drop in the methyl donor betaine (trimethylglycine). So this study shows that people who are poor methylators may need to watch their nicotinic acid and nicotinamide intake, or supplement TMG/other methyl donors.