Gastroesophageal Reflux Disease (GERD): This condition can cause increased saliva production as the body tries to neutralize stomach acid. Additionally, GERD can lead to inflammation and irritation in the respiratory tract, resulting in increased phlegm production.

Asthma or Chronic Obstructive Pulmonary Disease (COPD): Both conditions can cause increased phlegm production due to airway inflammation and irritation. Some medications used to treat these conditions, such as bronchodilators, can also increase saliva production.
Allergies: Seasonal or environmental allergies can cause increased phlegm production due to postnasal drip and sinus congestion. Some allergy medications, such as antihistamines, can also increase saliva production.
Medication side effects: Certain medications, such as anticholinesterases, cholinergic agonists, and some antidepressants, can increase saliva production as a side effect.

Cystic Fibrosis: Although rare, cystic fibrosis is a life-threatening condition that can cause increased phlegm production due to abnormal mucus production in the lungs. Early diagnosis and treatment are crucial to prevent long-term complications.
Tuberculosis: This infectious disease can cause increased phlegm production and should not be missed due to its potential for serious complications and transmission to others.
Lung Cancer: Although less likely, lung cancer can cause increased phlegm production and should be considered in patients with a history of smoking or other risk factors.

Sjögren's Syndrome: This autoimmune disorder can cause increased saliva production due to inflammation in the salivary glands.
Hypothyroidism: Some cases of hypothyroidism can cause increased saliva production and phlegm due to decreased thyroid hormone levels.
Neurological disorders: Certain neurological conditions, such as Parkinson's disease or cerebral palsy, can cause increased saliva production due to impaired swallowing and oral motor function.