Headache's are common symptoms of altitude sickness. It's a sign your brain is not getting enough oxygen.
As with lots of mountain issues it's down to judgement. If the headache is impairing your ability to perform at altitude, then yes, it's dangerous. If you can't concentrate on what your doing then your a danger to yourself and others.
Bear in mind, you're not necessarily the best person to make the decision on wether to go on or go down. If your alone, bear on the side of caution, head down. If your in a group, tell the other people how you feel so they can keep an eye on you.
A headache on it's own is unlikely to kill you. Your unlikely to simply have an animism and keel over.
Other symptoms of altitude sickness listed on the NHS web site are:
- nausea
- dizziness
- exhaustion
If you get the below issues, get the hell down ASAP, call mountain rescue, whatever just get down now!
- a bubbling sound in the chest
- coughing up pink, frothy liquid
- clumsiness and difficulty walking
- confusion leading to loss of consciousness
- hallucinations
- a blue tinge to the skin
These are symptoms of High altitude pulmonary oedema (HAPE). This means you effectively drown in your own fluids. Not good...
Obviously, the main way to avoid these issues is to acclimatise properly