likely its comes from mint oil they use, oils can be very irritating to the gums. apparently peppermint contains menthol, could be behind the irritation it can cause? apparently spearmint barely contains any menthol. it seems its likely the menthol in pepperminted toothpaste that cause irritation. could not figure out why some off-brand ones are causing it. just checked on some reviews of toothpaste with mint, its mostly the mint+ menthol likely peppermint. since spearmint taste much milder than peppermint (which gives you that fresh cool feeling)