It's not dangerous for an adult to drink some leftover baby formula as long as you're not relying just on the baby formula for adult nutrition.
Simply drinking some leftover baby formula won't harm you as an adult.
It is safe for an adult to drink baby formula as long as the adult is also eating a healthy balanced diet and not relying solely on the baby formula for nutrition.
Adults bodies need more nutrients and whole foods to survive and stay healthy and baby formula is not enough to sustain an adult.
But as long as the adult is eating a healthy balanced diet of other foods, then it's safe for an adult to drink some baby formula.
However it's best to not drink the baby formula everyday or more than a glass or few cups of baby formula per day as an adult.
I've always consumed my baby's leftover baby formula to keep from wasting it and I've been fine.
Just don't make the baby formula your sole source of nutrition or try to survive just on the baby formula alone.
Consuming just baby formula or consuming too much baby formula as an adult could lead to some severe health issues, but simply drinking some baby formula on occasion won't harm you.