Cystic fibrosis (CF) causes thick, sticky mucus to build up in the lungs, digestive tract, and other organs, producing symptoms that can range from a persistent cough and frequent lung infections to poor growth, greasy stools, and infertility. Most people are diagnosed in infancy through newborn screening, but milder forms can go undetected into adulthood. The specific symptoms and their severity vary widely from person to person.
Why CF Affects So Many Organs
CF is caused by a defective protein that normally moves salt and water across the surface of cells lining the lungs, digestive tract, reproductive organs, and sweat glands. When this protein doesn’t work properly, the thin layer of fluid that keeps these surfaces moist becomes depleted. Mucus that would normally flow freely turns thick and sticky, clogging airways, blocking digestive enzymes from reaching the intestines, and disrupting the function of several organ systems at once.
Early Signs in Babies and Young Children
Many parents first notice something is off when their baby tastes salty when kissed. That saltiness comes from abnormally high chloride levels in sweat, one of the hallmark signs of CF. Other early indicators include poor weight gain despite a good appetite, constant coughing or wheezing, thick mucus, and stools that are greasy, pale, bulky, and foul-smelling.
Some newborns present with a bowel obstruction called meconium ileus, where thickened stool blocks the intestine in the first days of life. This occurs exclusively in babies with CF and often leads to an immediate diagnosis. In the United States and many other countries, newborn screening blood tests now catch most cases before symptoms become obvious.
Lung and Sinus Symptoms
The lungs are the most commonly affected organ. Thick mucus clogs the airways, creating an environment where bacteria thrive. The result is a cycle of chronic infection and inflammation that gradually damages lung tissue. Day to day, this shows up as a persistent cough that produces mucus (sometimes tinged with blood), wheezing, shortness of breath, and frequent chest congestion.
People with CF experience periodic flare-ups known as pulmonary exacerbations, marked by worsening cough, increased mucus production, a change in mucus color, and greater difficulty breathing. These episodes often require intensified treatment and sometimes hospitalization. Chronic sinus infections and nasal polyps are also common, causing facial pressure, congestion, and loss of smell.
Coughing or spitting up blood can occur when inflammation erodes a blood vessel in the airway. While small amounts of blood-streaked mucus are relatively common, larger bleeds are a medical emergency.
Digestive and Nutritional Problems
About 85% of people with CF have pancreatic insufficiency, meaning the pancreas can’t release enough digestive enzymes into the intestine. Without these enzymes, the body struggles to break down and absorb fat, protein, and fat-soluble vitamins (A, D, E, and K). This leads to the hallmark greasy, foul-smelling stools, along with bloating, abdominal pain, and poor nutrient absorption.
In children, the nutritional toll is significant. Poor fat absorption stunts growth and weight gain, and deficiencies in essential fatty acids can contribute to skin problems, hair thinning, and further growth failure. Vitamin K deficiency, if severe, can cause unusual bruising or bleeding. Zinc deficiency is another concern in young children who aren’t growing well.
Intestinal blockages can happen at any age. In older children and adults, a condition called distal intestinal obstruction syndrome occurs when thickened intestinal contents partially or fully block the junction between the small and large intestine, causing cramping, bloating, and vomiting. Chronic constipation, acid reflux, and recurrent pancreatitis are also part of the picture.
CF-Related Diabetes and Liver Disease
As people with CF live longer, complications involving the pancreas’s hormone-producing cells and the liver have become more prominent. CF-related diabetes develops when thick secretions damage the insulin-producing parts of the pancreas. It shares features with both type 1 and type 2 diabetes, and its onset often coincides with worsening lung function, unintended weight loss, and increased fatigue. It typically appears in the teens or twenties.
Liver disease affects a smaller but significant number of people with CF, most often developing during childhood. Thick bile blocks the small ducts within the liver, which can progress to scarring (cirrhosis) and elevated pressure in the liver’s blood vessels. Signs include an enlarged liver, poor growth, low body weight, and in severe cases, bleeding complications from vitamin K deficiency. Liver disease also accelerates bone thinning, which already tends to be a problem in CF due to chronic inflammation and poor vitamin D absorption.
Reproductive Effects
Roughly 97% to 98% of men with CF are infertile. The same thick secretions that affect the lungs and gut also block or prevent normal development of the tubes that carry sperm. The testes typically produce sperm normally, but it has no way to reach the ejaculate. For many men, infertility discovered during fertility testing is actually the first clue that they have CF, particularly in milder cases. Assisted reproduction techniques can often help men with CF father biological children.
Women with CF are generally fertile, though conceiving can take longer. Thickened cervical mucus may make it harder for sperm to reach the egg, and poor nutritional status or low body weight can delay puberty and disrupt menstrual cycles. Some women with CF underestimate their fertility and skip reliable birth control, leading to unplanned pregnancies.
Atypical CF Diagnosed in Adulthood
Not everyone with CF fits the classic picture. People with milder genetic mutations may have symptoms in only one organ system, and these can be subtle enough to escape diagnosis for decades. Adults with atypical CF might be told they have poorly controlled asthma, chronic obstructive pulmonary disease, or recurring sinus infections without anyone connecting the dots. On the digestive side, chronic constipation, unexplained diarrhea, or recurrent bouts of pancreatitis with little or no lung involvement can be the primary presentation.
Other less obvious signs include unexplained electrolyte imbalances (particularly during hot weather or after exercise), skin that wrinkles quickly when wet, nutritional deficiencies causing dermatitis or anemia, and the fingernail clubbing that comes with chronic low oxygen levels. Over time, people with atypical CF often develop additional symptoms or realize that health problems they’ve had for years were actually related to CF all along.
How CF Is Confirmed
The sweat test remains the gold standard for diagnosis. A small amount of sweat is collected from the skin, and the chloride concentration is measured. A level of 60 mmol/L or higher confirms CF. Results below 30 mmol/L make CF unlikely. Values between 30 and 59 mmol/L fall in an intermediate range that calls for repeat testing or genetic analysis. People with atypical CF may have borderline or even normal sweat chloride levels, which is one reason milder cases get missed.
Genetic testing can identify the specific mutations responsible and help predict which organs are most likely to be affected. More than 2,000 mutations in the CF gene have been identified, which partly explains the wide spectrum of severity. Based on current Cystic Fibrosis Foundation registry data, the median predicted survival for someone born with CF between 2020 and 2024 is 65 years, a dramatic improvement from previous decades driven largely by newer therapies that target the underlying protein defect.