5. Diarrhea
Diarrhea can lead to dehydration simply because it causes you to constantly empty your bowels and thereby lose a huge amount of water and nutrients.
This is actually the point at which diarrhea can become dangerous. So, if you have diarrhea, then make sure to keep drinking. If you literally find that you can’t keep anything down, then this is the time when it becomes highly important that you go and see your doctor.
Likewise, the same is true if you are being sick a lot. Keep drinking and look for ways to try and reduce dehydration. Likewise, avoid things that will worsen it such as the aforementioned caffeine.