The foregoing refers to the effects expected in a normal person. Reference has already been made to anaemia, including that produced by smoking. Any chest disease adversely influences the oxygen uptake. Physical activity increase the need for oxygen, as does cold which in itself clouds mental judgement. At 18,000ft the outside temperature is 20 C, and shivering can double oxygen consumption.
All sedative drugs increase the effects of hypoxia.
These include tranquillisers such as Valium. Antihistamines for hay fever and colds, and of course alcohol. Alcohol is a depressant drug and clouds judgement. A really heavy night at the bar can leave a significant alcohol level during the following morning. The process of acclimatisation or becoming used to low oxygen levels takes weeks of months, and cannot be achieved even in a Silver duration flight.