Our expert says:
why do you want to give it to her - to keep her calm? Newer studies showed that anti-histamines do not work well for babies under 1 year as sedative and may sometimes have the opposite effect. Rather ask your doctor to prescribe something according to her age and weight. Put a drop of pediatric illeadin nosedrops in each nostril just before you fly - it helps to prevent ear ache.
Take something with for her to suck on. You can use homeopathic medication like Rescue remedy ( 4 drops diluted in a little water) as an alternative to antihistamines or sedatives.
See these tips from a flying mom: http://ihome.ust.hk/~lbcaplan/hkbb/flyingbb.html
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