Overweight
This fruit is a typically healthy one but the accumulation of calories can cause significant weight gain, causing more harm than good.
Hyperkalemia
The potassium in bananas is good for many reasons, including promoting heart health, but if an excessively high amount enters the body it may lead to hyperkalemia. This is a health condition where the patient suffers from muscle weakness, temporary paralysis, and irregular heartbeats.
Headaches
There have been multiple instances where people who ate too many bananas complained about headaches. This is due to the amino acids in bananas that dilate blood vessels. Eating ripe yellow bananas puts you more at risk of having this side effect, compared to green unripe ones.
Sleepiness
The amino acid tryptophan in bananas converts to serotonin when digested, which improves your ability to sleep. If serotonin is produced in excess due to overconsumption, it may lead to a serious condition of sleepiness.
Reference:
[1] https://www.healthline.com/nutrition/bananas-calories-carbs
[2] https://www.healthline.com/nutrition/bananas-and-weight
[3] https://www.webmd.com/food-recipes/health-benefits-bananas
[4] https://www.healthline.com/nutrition/foods/bananas
[5] https://medlineplus.gov/ency/article/002332.htm