Safety in Flight

The use of any sort of CNS depressant such as diazepam causes longer reaction times and slowed thinking, which during a flight can put a passenger at significant risk of not being able to act in a manner that could save their life in the event of a safety-critical incident. Furthermore, there may be a paradoxical increase in aggression reported by patients taking benzodiazepines with the potential to put other occupants of the aircraft at risk. Benzodiazepines added to alcohol consumption cause an increase in the risk posed by many of the points above, and many (nervous) flyers will consume alcohol in the terminal before boarding and during their flight, despite medical advice to refrain from doing so. It is also worth noting that the BNF states that Benzodiazepines should be avoided in phobic states.

Risk of Thrombosis

The use of any sort of CNS depressant has the potential to increase the risk of Deep Vein Thrombosis. Such drugs may induce non-REM sleep which tends to be of a type where the person does not move in their sleep and therefore increases the possibility of sitting without moving for more than four hours at a time (the amount of time which has been shown to increase the risk of developing DVT whether in an aeroplane or elsewhere).

Respiratory Depression

The sedating effects have the potential to cause respiratory depression, resulting in a drop in oxygen saturation.

Legalities in Foreign Domains

In some countries, it is illegal to import such drugs, e.g. in the Middle East, and so passengers will need to use a different strategy for the homeward-bound journey and/or any subsequent legs of a journey. 

Standard GP indemnity does not treatments initiated outside the UK, so your GP would only be insured for medicines administered within the UK. 

NICE 

Guidelines suggest that medication should not be used for mild/self-limiting mental health disorders and that there is a risk of addiction from inappropriate use. In more significant anxiety-related states, benzodiazepines, sedating antihistamines or antipsychotics should not be prescribed. Benzodiazepines are only advised for short-term use for a crisis in generalised anxiety disorder, i.e. acute anxiety emergencies. In such a scenario, an individual would not be fit to fly (due to the potential risk of adversely affecting the completion of the flight without diversion). Fear of flying in isolation is not a generalised anxiety disorder. There is also a possible risk of early dementia in users (though it is currently unclear if this risk is in regular users only, or includes occasional use).