Acute mountain sickness starts as a headache with nausea, poor appetite or broken sleep. It is common above 3,000 m and usually harmless — as long as you stop going up until it resolves.
The three rules that prevent most cases: gain no more than 500 m of sleeping altitude per day above 3,000 m, take a rest day every 1,000 m, and walk high during the day before descending to sleep.
Hydration and pace matter more than fitness. Fit trekkers get AMS more often than slow ones, because they climb faster than their bodies adapt.
Carry a pulse oximeter, know the descent options on your route, and treat worsening symptoms as a descent decision rather than a discussion. Every one of our itineraries builds in acclimatisation days for this reason.