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Firstly, the obvious one being excessive daytime sleepiness. It is the most crippling factor of the condition, and the primary one.
Secondly, if you have Narcolepsy with Cataplexy then I would say Cataplexy is the next worst symptom.
Overall there are a lot of minor symptoms which result from both the medication and the disorder, but these are many in number and vary for the individual so it would not be pertinent to list them here. Finding a balance in medication and sleep hygiene is crucial for minimising these symptoms which can reach high severity levels if ignored and diet and lifestyle changes aren't made.