My Nephew Took 2.5ml Of AllerphenH For His Cold....he Is

My nephew took 2.5ml of allerphenH for his cold....he is 13 months of age...10.5kg weight...he took that med six hours ago...since thn he is in a drowsy state but unable to sleep....all he wants is someone to hold him nd keep standing....why is this happening???He cries really badly whenever we try to put him to bed...he,sleeps only whn he is held by his mother....