here's what i found about b12 with ibs/diarrhea--Kinda sounds like a vicious cycle lol but certainly with IBS-D you should get tested for vitamin deficiencies because so much of our food nutrients are not absorbed into our systems:

Vitamin B12 may not directly help with general IBS diarrhea, but it can be beneficial if you have a B12 deficiency, which can mimic IBS symptoms like diarrhea, stomach pain, and fatigue. IBS can impair nutrient absorption, making B12 deficiency more common in some individuals with the condition. Supplementing with B12 might be necessary in these cases but should be discussed with a healthcare provider to confirm a deficiency and determine the best form and dosage.
Why B12 might be relevant to IBS:
Mimics symptoms: A B12 deficiency can cause symptoms similar to IBS, such as diarrhea, cramping, and fatigue.
Impaired absorption: Conditions that cause IBS can also reduce the body's ability to absorb B12 from food, leading to deficiency.
Higher risk: Studies show that people with IBS have a higher risk of B12 deficiency.
When supplementation may be helpful:
If you have IBS and experience symptoms like chronic diarrhea, fatigue, or neurological issues, a healthcare provider might recommend a blood test to check your B12 levels.
If a deficiency is confirmed, supplementation can help restore levels.
How to approach B12 supplementation:
Consult a doctor:
Always talk to a healthcare provider before starting any supplements, especially if you have a chronic condition like IBS.
Consider testing:
Your doctor can order tests to determine if you are truly deficient in B12.
Discuss absorption issues:
If your B12 deficiency is due to poor absorption, your doctor might suggest alternative forms of B12 that bypass the digestive system, such as intra-oral (under the tongue) or injectable forms.