Bitter cucumbers are safe to eat.
The actual cause of bitterness in cucumbers and other gourds, like zucchini, is due to a compound known as cucurbitacin.
This compound is always present in cucumbers, but the levels change based on the growing process of the vegetables.
The higher the level of cucurbitacin, the more bitter the cucumber.
Nothing is harmful about eating bitter cucumbers although you may not enjoy them.