Irritable Bowel Syndrome (IBS): Given the chronic nature of the diarrhea (3 weeks) and lack of relief with common over-the-counter medications, IBS is a strong consideration. It's a common condition characterized by abdominal pain and changes in bowel movements without an identifiable cause.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic diarrhea. The fact that common medications haven't provided relief suggests a more serious underlying condition could be at play.
Bacterial Overgrowth (SIBO): Small intestine bacterial overgrowth can lead to chronic diarrhea, bloating, and abdominal pain. It's more common in individuals with certain anatomical or functional abnormalities of the small intestine.
Giardiasis: An infection caused by the Giardia parasite, which can lead to chronic diarrhea if not treated properly. Travel history, exposure to contaminated water, or immunocompromised status could increase the likelihood.
Lactose Intolerance or Other Food Intolerances: Though typically presenting with more acute symptoms, chronic exposure to lactose or other intolerable substances could lead to persistent diarrhea in susceptible individuals.

Colorectal Cancer: Although less common in younger individuals, colorectal cancer can present with chronic diarrhea among other symptoms. It's crucial to rule out this potentially life-threatening condition, especially if there's a family history or other risk factors.
Thyroid Disorders: Hyperthyroidism can cause chronic diarrhea due to the increased metabolic rate. This diagnosis is critical not to miss due to the potential for significant morbidity if left untreated.
Infectious Diarrhea (e.g., Clostridioides difficile): Especially if the patient has recently been hospitalized or exposed to antibiotics, C. difficile infection is a critical diagnosis to consider due to its potential severity and resistance to standard treatments.
HIV/AIDS: Chronic diarrhea can be a presenting symptom of HIV/AIDS, particularly in advanced stages. Given the significant implications for treatment and prognosis, this diagnosis should not be overlooked.

Whipple's Disease: A rare, systemic bacterial infection that can cause chronic diarrhea, among other symptoms like weight loss and joint pain. It's essential to consider in cases where common diagnoses have been ruled out.
Short Bowel Syndrome: Typically resulting from significant surgical resection of the small intestine, this condition can lead to chronic diarrhea due to the reduced absorptive surface area.
VIPoma: A rare endocrine tumor that secretes vasoactive intestinal peptide (VIP), leading to a syndrome characterized by large-volume diarrhea, hypokalemia, and achlorhydria.
Carcinoid Syndrome: Although more commonly associated with flushing and heart valve abnormalities, carcinoid tumors can also cause diarrhea, particularly if they have metastasized to the liver.