Irritable Bowel Syndrome (IBS): Characterized by abdominal pain, bloating, and alterations in bowel habits, IBS is a common condition that can cause constipation one day and diarrhea the next due to its unpredictable nature and the variability in bowel movements it causes.

Gastroesophageal Reflux Disease (GERD): Although primarily known for causing heartburn, GERD can lead to changes in bowel habits due to the reflux of stomach contents into the esophagus, potentially irritating the digestive system and causing alternating constipation and diarrhea.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause a wide range of gastrointestinal symptoms, including alternating constipation and diarrhea, due to inflammation and damage to the bowel lining.
Food Intolerances: Intolerances to certain foods, such as lactose, gluten, or fructose, can lead to gastrointestinal symptoms, including constipation and diarrhea, depending on the individual's sensitivity and the amount of offending food consumed.

Infectious Gastroenteritis: While often presenting with diarrhea, some infections can cause an initial phase of constipation before leading to diarrhea, making it crucial to consider, especially in cases of recent travel, food poisoning, or exposure to someone with a similar illness.
Colorectal Cancer: Although less common, colorectal cancer can cause changes in bowel habits, including alternating constipation and diarrhea, especially if the tumor obstructs or partially blocks the bowel.
Neurological Disorders: Conditions like Parkinson's disease, multiple sclerosis, or spinal cord injuries can affect bowel function, leading to unpredictable bowel movements due to the impact on the nerves controlling the bowel.

Gastroparesis: A condition where the stomach takes too long to empty its contents, gastroparesis can lead to unpredictable bowel movements, including both constipation and diarrhea, due to the irregular movement of food through the digestive system.
Small Intestine Bacterial Overgrowth (SIBO): An overgrowth of bacteria in the small intestine can lead to a variety of gastrointestinal symptoms, including alternating constipation and diarrhea, due to the malabsorption of nutrients and the production of gas by the bacteria.
Hormonal Imbalances: Certain hormonal conditions, such as thyroid disorders, can affect bowel function, leading to changes in bowel habits, including constipation and diarrhea, although these are less common causes.