Everyone needs a certain amount of niacin -- from food or supplements -- for the body to function normally. This amount is called the dietary reference intake (DRI), a term that is replacing the older and more familiar RDA (recommended daily allowance). For niacin, the DRIs vary with age and other factors:
Women: 14 milligrams daily
Women (pregnant): 18 milligrams daily
Women (breastfeeding): 17 milligrams daily
Maximum daily intake for adults of all ages: 35 milligrams daily
Most people can get the amount of niacin they need by eating a healthy diet. 
According to the U.S. Food and Drug Administration (FDA) classifies oral niacin under Pregnancy Category C which means that it has not been fully studied on pregnant humans and animals. Since there are no adequate studies to determine the possible effects of niacin (at high doses) on pregnancy and the unborn child; caution is advised when using this drug. On the other hand, doses not exceeding the recommended dietary allowance for pregnant women are considered safe.
It is generally advised that women taking high doses of niacin for treatment of high “bad cholesterol” level should stop taking the high levels of this vitamin throughout pregnancy. Meanwhile, women taking niacin for high triglycerides may need to continue taking the vitamin since pregnancy increases the risk of complications due to high triglycerides(such as pancreatitis). Discuss with your healthcare provider whether it is safe for you to continue taking the drug during pregnancy. 
Niacin During Pregnancy
Niacin deficiency is also a concern during pregnancy, and it is relatively common according to a study published in 2002 in the "Journal of the American College of Nutrition." The study found that deficiencies in niacin, thiamine, vitamin B-12, vitamin B-6 and vitamin A were common in all three trimesters of pregnancy even with vitamin supplementation. 
Can Breastfeeding Women Take Niacin?
In general, breastfeeding women can safely take low doses of niacin. However, breastfeeding women are advised to discuss with their healthcare provider the safety of this vitamin before initiating treatment as more studies are needed to determine the safety of high dose-niacin for breastfeeding women and the infant.
Basically, niacin dosage not exceeding the recommended dietary allowance (RDA) for breastfeeding women of 35 mg/day is considered safe. 
1. Niacin (Vitamin B3) http://www.webmd.com/diet/supplement-guide-niacin#1
2. How Much Niacin Is Okay During Pregnancy? http://www.livestrong.com/article/441111-how-much-niacin-is-okay-during-pregnancy/
3. Niacin During Pregnancy & Breastfeeding http://www.drugsdb.com/sup/niacin/niacin-during-pregnancy-breastfeeding/
Dr Nicole Ng (MBBS) is a medical doctor with a passion in women's health and medical research