How Quickly Does a B12 Shot Work? Benefits & What to Expect Could the B12 (Vitamin B12) injection I received at 4pm be causing my insomnia, considering my history of Small Intestine Bacterial Overgrowth (SIBO) and malabsorption? How Long Do B12 Injections Last: 5 Ways to Extend Effects How should I manage a 33yearold patient with worsening anxiety, racing thoughts, low mood (3 10), grief, constipation, nocturnal urination, currently taking atomoxetine 40 mg daily (with occasional missed doses), vitamin D2 weekly,