Taking large doses of b12(approx. 5000 mcg) makes me feel weak/sleepy. what could be going wrong?

B12 dosing: Vitamin B12 is water soluble and thus high doses are eliminated from the body fairly efficiently. However, high doses like 5000 / d can indeed have side effects ranging from nausea and weakness to more specific side effects such as diarrhea, anxiety, skin irritations, insomnia, and irritability. Also, B12 deficiency does cause a range of neurologic problems, so a mixed picture can present itself.

Need evaluation: No one needs to take 5000 mcg of B12. The daily requirement is about 1-5 mcg. There is likely to be some other cause for your symptoms but it is not feasible to figure that out, without more history, physical examination and some tests. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, low fat milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco or alcohol in any form. Practice safe sex.