Cardiovascular Benefits
Niacin is the only form of vitamin B3 that improves cholesterol levels. It is involved in chemical reactions that metabolize fats in the body. It affects the body's cholesterol balance in a similarly to many statin drugs, although through a different mechanism. Niacin blocks the production of VLDL (a precursor to the "bad" LDL cholesterol), and increases the level of HDL ("good" cholesterol) in the body.  In addition to changing the lipid profile of the body, niacin also triggers cells to release histamine, a chemical responsible for dilating blood vessels. This affects both our internal blood vessels, and the blood vessels in our skin. When the blood vessels in the skin dilate, it causes a reddening that is commonly referred to as a "niacin flush".  It is through this dilatory effect that niacin can benefit the circulation in cardiovascular disease, as well as other diseases where circulation is compromised. For example, niacin can help to increase circulation in the extremities of diabetic people suffering from numbness due to reduced blood flow.  Interestingly, these processes of cardiovascular benefit are tied to the conversion of niacin to NADH, and thus are provided only by niacin supplementation, and not by using the non-flushing forms of niacin (niacinamide or IHN).

Metabolism
Niacin is involved in a number of different metabolic processes in the body, and it even plays a part in the production of stomach acid and bile for the initial digestion of food. It affects blood sugar by acting as a component of glucose tolerance factor, one of the blood-sugar regulatory molecules in the body.  In this way, niacin is a therapeutic agent for diabetes. It also helps with the digestion of fats, carbohydrates and protein because niacin is converted to NADH in the body. NADH increases the rate of energy-producing chemical reactions in the body and helps to repair damaged DNA.

Nervous System and Mental Health
Vitamin B3 is an important vitamin for the proper functioning of the nervous system, and has been well researched for its ability to improve the outcomes of mental illness, such as schizophrenia.  It is also used by many practitioners to help treat depression and anxiety.

Allergies
Niacin can also help allergy sufferers by depleting the body's histamine stores during the initial flush response.  Histamine release leads to allergic symptoms like runny nose and itchy, watery eyes. Thus, reducing the body's stores of this allergy-producing chemical, by using them up in the flush response, can help to reduce these common symptoms in allergy sufferers.

Niacin Deficiency
Niacin deficiency is known as "pellagra", which is a condition that appears with symptoms of: diarrhea, dementia, dermatitis (skin rashes), and eventually results in death ("the 4 D's"). It can also cause swollen tongue, an enlarged heart, and extreme sensitivity to sunlight.

Co-Supplementation
All of the B vitamins are cofactors in a series of reactions and thus, work together in the body. Because of this, they are best supplemented together in a B complex. This is why most B complex supplements include a small dose of niacin.  B vitamins are water-soluble, which means that any extra remaining in the body after large supplemental doses is easily flushed out in the urine. This makes it difficult to overdose on B vitamins in the long term, but also means that it is best to take lower dosages of your B complex several times a day rather than one large dose. Taking niacin with inositol (like IHN) can help to reduce the skin-flushing side effects.

How can I get more Niacin?
In general, it is a good idea to take niacin along with other B vitamins in a B complex supplement.

If additional supplementation of niacin is needed, it can be taken in addition to this B complex to avoid any depletion of other B vitamins.  Spray or sublingual forms of B vitamins are easily absorbed, as are capsules. If your digestion is healthy, then the tablet form is usually suitable for most people. There are time-release formulations available to create a more steady release of niacin throughout the day. The dosage recommendations for niacin vary widely depending on the application. Daily supplementation for general health is usually suggested at between 40 and 300mg of niacin per day. When attempting to achieve cardiovascular and neurological benefits, most studies use between 1,000 and 3,000mg (1-3g) per day. It is important to note that these higher dosages can be associated with toxicity. Because of this, it is recommended that people taking high doses of niacin in order to modify their cholesterol levels should have frequent blood tests to monitor liver function. In general, if you want to do high dose niacin therapy, you should discuss it with your physician first, and have them monitor you throughout treatment.

Niacin Toxicity
Immediate toxicity of niacin appears as nausea, dizziness, itching, vomiting, and dermatological conditions such as extreme flushing and rashes. It is important to note that high doses of niacin have also been associated with indigestion, high blood sugar, heart arrhythmias, and even liver damage. It is a good idea to be monitored by your healthcare practitioner while taking higher doses of niacin regularly.

Important Information
The elderly and those with impaired digestion are particularly prone to deficiency of all B vitamins, so it is often a good idea to supplement with a B complex in these individuals. Niacin (nicotinic acid) causes a transient skin flushing caused by the release of histamine. This flushing can also cause harmless tingling or burning sensations in the skin. These effects are worsened by taking niacin with alcohol or hot liquids or while on some antibiotics. Taking niacin on a full stomach, with cold water or with inositol can help to minimize these effects.  Individuals who cannot tolerate the niacin flush response can try supplementing with inositol hexaniciacinate (IHN) or niacinamide, both of which do not cause the flush response. Time-release forms of B3 may increase the risk of liver toxicity, so using them with higher dosages is particularly discouraged.  Caution should be used when supplementing niacin if you have a stomach ulcer, active bleeding, or liver or kidney disease. Vitamin B3 should not be used by pregnant or nursing mothers as it may cause harm to the baby. Vitamin B3 can interact with: blood thinners, blood pressure and heart medications, sleeping pills, estrogen, alcohol and sulfonamide antibiotics.