The Reddit user, Thrwydad, said his 12-month-old boys are "completely identical" and sometimes he and his wife can't tell them apart.
His son Aaron has serious health issues and missing his medication could lead to significant consequences.
"We have been drawing on Adam's [Aaron's twin brother] arm every day with permanent marker (Aaron has very sensitive skin and we don't want to irritate it)," the dad explained.
"This backfired when the twins were at grandma's house and after a bath it rubbed off. Grandma accidentally gave Aaron's medication to Adam," he wrote.
"Adam ended up very sick and Aaron in hospital for a week. This is a serious extreme situation and I don't want to take this risk again. We have had other mix ups with the boys but it has never wound up this bad."
The dad explained he and his wife have tried other things such as nail polish, bracelets, anklets or cutting one of their boy's hair, but they are after a more "permanent" solution.
"I read online about someone tattooing a single dot on one of their twins and also about piercing ears," he continued.
"I really hate to say it but we are here. I need to do something permanent.
"I could have lost both my sons last weekend and I'm desperate. I am only after permanent solutions.
"Believe me when I say we have tried everything else before and it didn't work. Please help me. I don't want to lose my boy."
Reddit users replied with comforting messages, some explaining they know parents of twins who have got tattoos on the bottom of their toddler's foot.
Many agreed that a small tattoo spot would be better than getting a piercing, which is harder to cover up and people may "judge".
There were only a few people commenting who believed ear piercing would be the better option.
However those who agreed on the tattoo said it would be better to get it done by their pedatrician or in a medical setting.