Can I pass a drug test in 12 days if I'm drinking 4litters of water and taking 1000mg of niacin a day and I stopped smoking two days ago?

Likely to pass. Nicotine and cotinine should be out of your system in 14 days. Niacin is not likely to help. Now that you are off tobacco. do not ever use it again. For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex.