Irritable Bowel Syndrome (IBS): Given the chronic nature of the symptoms (1.5 months) without any mention of blood in the stool, weight loss, or systemic symptoms, IBS is a common cause of chronic loose stools in adults.
Infectious Gastroenteritis: Although less likely to persist for 1.5 months, some viral or bacterial infections can cause prolonged gastrointestinal symptoms.

Lactose Intolerance: This condition can cause chronic loose stools, especially if the patient consumes lactose-containing products regularly.
Small Intestine Bacterial Overgrowth (SIBO): SIBO can lead to chronic diarrhea due to the overgrowth of bacteria in the small intestine.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can present with chronic diarrhea, although they often include other symptoms like blood in the stool and weight loss.
Celiac Disease: An autoimmune reaction to gluten can cause chronic diarrhea among other symptoms.

Colorectal Cancer: Although less common in a 40-year-old, it's crucial not to miss this diagnosis, especially if there's a family history or other alarming symptoms like weight loss or blood in the stool.
Giardiasis: Infection with Giardia lamblia can cause chronic diarrhea and is particularly important to consider if the patient has been traveling or exposed to contaminated water.
Thyroid Disorders: Hyperthyroidism can cause chronic diarrhea due to increased motility of the gastrointestinal tract.
Medication Side Effects: Certain medications, such as antibiotics, antacids, and laxatives, can cause chronic diarrhea as a side effect.

Whipple's Disease: A rare, systemic bacterial infection that can cause chronic diarrhea among other symptoms.
Short Bowel Syndrome: A condition that can arise after a significant portion of the small intestine is removed, leading to chronic diarrhea and malabsorption.
VIPoma: A rare tumor that secretes vasoactive intestinal peptide (VIP), leading to chronic, watery diarrhea.

Can you describe your typical bowel movements? How often do you have loose stools?
Have you noticed any blood in your stool or black, tarry stools?
Have you experienced any weight loss, fever, or abdominal pain?
Do you have any family history of gastrointestinal diseases or cancers?
Have you recently traveled or been exposed to anyone with similar symptoms?
What is your diet like? Do you consume lactose or gluten regularly?
Have you started any new medications recently?
Do you have any other symptoms like fatigue, joint pains, or skin rashes?