Close friend of mine got into deep trouble on this one. Look, it depends on whether it is Type One or Type two. Type one is where daily insulin jabs have to be taken and is more serious insofar as control is more difficult. Licencing Authorities are less likely to allow you to fly with this. Type two is very common, linked to overweightness & the dreadful diets we succumb to. It is treatable & controlled with medication and/or diet alone. Licencing Authorities will want to satisfy themselves that the sufferer is controlling and will, probably allow you to continue to maintain your licence with what is known as a MOL restriction. (Multi Crew Ops) whereby you can fly only in a multicrew scenario and , of course,with no others in that crew who suffer with the same condition. My mate, in a ME airline, was diagnosed by his Medical Dept with Type 2.He was grounded, subjected to all sorts of additional tests (Blood, stress ECG, Eyeball pressure etc) and placed on medication (over recommended doseage) which left him in a dreadful state. UK medics suggested he just had very high sugar levels and was NOT diabetic. His ME CAA approved medic passed him after medication was abandoned ( it was making him very ill). Having taken issue with his Company's medical department, the loss of face issue kicked in and he was suspended after the Company Doc got very upset ! My colleague pretended to go back on the medication, lost a couple of pounds in the gym, signed up to regular monitoring at a local Hospital & smiling Doc declared that the medication was clearly working (he was not taking any ) & restored his Class One with the MOL restriction ! He just retired, dropped a further two stone, cut back on his favourite tipple & walks everywhere. Never been fitter. The medication, prescribed by the Airline Doc, would have killed him (at one point pissing black tar with severe kidney pain) was ceremoniously thrown down the toilet.(The medication, not the airline Doc) .UKCAA has many Airline Pillots flying continuously with Type 2 on MOL restricted licences.Mid East, I don't know.Hope this is helpful and that you or someone you are asking about gets the correct diagnosis & obtains the level of care appropriate to the level of the condition.