Update: I’ve written a much more in-depth post on this topic, you can read it here!
Depending on concentration and bioavailability, niacinamide has been shown to have antimicrobial, anti-inflammatory, photo-protective, sebum reducing, and lightening effects when applied topically.
Niacinamide is the amide form of niacin.
The OH (oxygen and hydrogen, or hydroxyl) molecules have been replaced by an amine group – the nitrogen and two hydrogen molecules.
There are other topical forms of niacin, where the hydroxyl group has been replaced by a fatty acid – but their use is patented (NIA24 products).
Niacin isn’t often used topically because it causes prostaglandin release, which results in skin flushing and itching. Niacinamide and other forms of niacin don’t have this undesirable side-effect.
Niacinamide is quite stable, but in the right conditions can hydrolyze (breakdown in water) to niacin.
This is a concern for manufacturers who need to ensure long-term stability of their products, but should not be of particular worry for people using a product containing niacinamide and then another containing an acid (or vice-versa).
The reason being is that this hydrolysis takes a considerable amount of time. At a pH of 2 and 90°C it takes over 75 hours to convert half of the niacinamide in to niacin. Heat greatly speeds up chemical interactions, and at normal temperatures this translates in to weeks, if not months.
While it’s certainly true and possible that a small amount of niacin will form, for most people it won’t be enough to cause skin flushing.
If you do notice skin flushing (and tingling), then space the two products further apart. But don’t worry too much, the bulk of the product is still in niacinamide form and niacin is involved in the same pathway that niacinamide is.
Addendum: There’s another paper that focuses on the interaction of ascorbic acid and niacinamide which is often brought out in this discussion.
The important part though is often missing – the interaction is minimal, reversible, and irrelevant at physiological pH.
Again, this is more of a concern for cosmetic manufacturers and not consumers mixing products.