There are a lot of talk about niacin's anti-ageing, anti-depression, fat loss, yada yada, and since I have supplemented with it for very long on and off, as has a 70 year old family member of mine, I will share my experiences.
OBVIOUSLY, note: niacin here refers to nicotinic acid ONLY.
Why nicotinic acid? Because it is the most safe of all the niacin versions to take in high quantities and for long periods of time.
(1) Niacin and weight loss.
After over 2 months of daily supplementation with 500mg(first month)-1g(next month) every single day, no effect on weight was observed in a 70 year old, overweight man.
(2) Niacin and depression.
Approx. two weeks of daily supplementation with 500mg+ (up to multiple grams) produced only occassional, very short-lived (10-30min) mild euphoria. Niacin certainly was not an anti-depressant, and I have a severe case of "depression".
(3) Niacin and exercise ability.
Niacin CAN slightly increase exercise performance but only sometimes. The problem is that although it only has a very mild effect on raising blood sugar (approx. 10%), this can sometimes cause dizziness-like symptoms, which makes exercising significantly harder.
(4) Niacin and anabolism (etc) due to growth hormone increase.
Again, no muscle-enhancing or fat-burning, or better recovery, was experienced in the 70 year old who continuously supplemented with it.
It is said that niacin should be administered slightly before an exercise to increase growth hormone secretion and I have done this many times but it has never produced any noticeable effect in myself.
(5) Niacin and anxiety.
Personally, niacin is very great for anxiety--this is why I've kept buying it after I originally tried it. It produces a great calm which lasts for 30 minutes to an hour, without noticeable sedation.
(6) Niacin and better blood flow.
The 70 year old family member who has supplemented it for a long time and still continues with it has reported that it may have increased blood flow to the feet, but I myself have experienced that sometimes it can cause symptoms like cold hands/fingers, which seems to run contrary to increasing blood flow; though this problem of cold hands and fingers appears to be rare.
(7) Niacin and erections.
It appears that niacin has a random but noticeable effect on libido, especially after frequent supplementation with myself. I am one of the anti-sex people so trust me on this, and it is a side-effect I dislike, though thankfully it is rare.
(8) Niacin for beauty (white skin, wrinkles, etc.)
From the 70 year old's own experience when trying it transdermally (yes, nicotinic acid does absorb through the skin), niacin does have a noticeable effect on wrinkles and whitening of the skin. It appears niacin is used in some beauty products for this very reason. The purpose of transdermal application in the 70 year old was purely for an antibiotic effect on a skin infection.
(9) Niacin and immune system.
Yes, both myself and the 70 year old have experienced that niacin does have a significant effect on combating infections, experientially much better than that of vitamin C and other things, and the effect is often experienced within the day.
Sadly, it appears that niacin is not good for all immune system compromises, such as skin infections as observed in the 70 year old.
(10) Niacin and constricted blood vessel type headaches.
Niacin can be used in for example caffeine withdrawal where it will, when it takes action, disolve the headache, but this anti-headache effect is short-lived and the real problem with caffeine withdrawal is the tiredness and depressive mood, not the headache itself, thus niacin is not useful for caffeine withdrawal (but sleep is).
(11) Niacin and energy.
Neither myself, nor the 70 year old, nor others I know who've supplemented with it, have experienced more energy with it. If there is an effect on energy, it's very minor.
(12) Niacin and niacin flush.
From my own experience as well as the 70 year old's experience, daily supplementation for two weeks, less or more, will reduce the chance of getting more niacin flushes. For example, the 70 year old who continuously is supplementing it at 1g no longer experiences the niacin flush.
If there are any questions, submit them and I will answer them.
As for the obvious "but this is anecdotal evidence!" nonsense: yes it is, but you are perfectly rational in trusting your own experiences contrary to test results, or trusting the experiences of other people contrary to test results. In fact, you do this all the time. You consume caffeine based upon other people's experiences, and so on. What is irrational is distrusting your own experiences.
And if you want to read pubmed articles, you are free to do that. I shouldn't have to link them.