Bitter tasting cucumbers are not really poisonous and will likely do you no harm when eating bitter cucumbers because I've eaten several cucumbers in my life and never have gotten sick.
But it's possible to get sick from eating bitter cucumbers if you eat too many of them or your immune system is compromised or you have some other digestive issues then the bitter cucumbers could be toxic to some people.
But if you're in good health then eating a few cucumbers every now and shouldn't do you any harm.