What's causing my depression and lethargy?

I think you should do some research on celiac disease. The reason I think you should look into it is because celiac disease can cause both depression and fatigue and many symptoms of the disease are caused by your body being unable to absorb B vitamins. I don't know why the drink would do it for you and not the pills, but you do absorb vitamins sublingually. Many people with celiac respond to sublingual (under the tongue) vitamin B, but not in pill form because their intestines are damaged from the disease. Do some research on it. There's no typical presentation, you can be underweight, normal weight, or overweight, some people have lots of symptoms and some people have none. The most common symptom is depression and fatigue is another very common one. It's an auto-immune disease that is caused by eating gluten. It's very common, 1 in 100 people have it. The cure is a gluten free diet. I hope this helps!