No side effect that I've noted. For me, works probably twice as well as any of the others, including Singular, Pulmicort, etc. (that bronchodilator in addition to the steroid is brilliant!) I have some tiredness, stuffiness, etc. off and on, but attribute that much more to the underlying allergies, asthma, etc. rather than the medication - certainly better on the meds than without. Does wonders for my actual breathing.