My daughter had a constant cough and she has had it since she was a baby. i've taken her to the doctors and they say there's nothing wrong with her.

Good to hear she's : Not a passive smoker! not knowing her age, i'd suggest you seek referral to a pediatric gastroenterologist to rule out gastroesophageal reflux. A pediatric pulmonologist would check for reactive airway disease ( mild asthma) & possibly allergy testing. An ENT doctor would check her nose, throat & ears. Video a coughing spell to show her pediatrician when you take her in.

Cough: Is a reflex that depends on an input signal to the brain and an executive signal from the brain that initiates the cough. If the brain has become too sensitive it can cause the cough without a physical cause. The clue often is whether she coughs in her sleep without wakening her. Sugar is a major cause of this because it causes secondary vitamin b deficiency that sensitizes the nervous mechanism.