Can Vitamin C and Niacinamide be used together ?

Niacinamide is an aromatic amide that is pretty stable molecule over a wide range of pH, whereas L-ascorbic acid is an unsaturated γ-lactone containing an enediol configuration which is susceptible to oxidation in aqueous solution that makes it a very good antioxidant. Let's explore different scenarios

1. Using a product with L-ascorbic acid and niacinamide in it

Concern - L-ascorbic acid causes acidic hydrolysis of niacinamide to niacin (niacin causes skin irritation) when formulated together in a product .

What you need to know

Acidic hydrolysis of amides occurs at very low pH (pH < 2) and high temperature (almost > 90 °C).

Kinetics (rate) of such hydrolysis reaction is highly temperature dependent, which means even if an L-ascorbic acid product is hypothetically formulated at pH 2 (which by the way never is the case and are generally formulated around pH 3.5) then at room temperature the rate of the reaction will be so slow that it will almost surpass the shelf life of the product for the significant conversion of niacinamide to niacin.

Conclusion

A well formulated product with L-ascorbic acid and niacinamide should be fine to use in most cases. The plausibility of niacin formation depends on the overall formulation and one can't predetermine it by looking at the INCI. L-ascorbic acid products themselves can sometime cause irritation because of the low pH. So it is very difficult to know whether niacin formation is actually causing the irritation.

Therefore, if you are using a well formulated product and are happy with the results continue to use it.

2. Niacinamide and L-ascorbic acid form a complex when present together and cancels each other out 

Therefore, won't negate the benefits of individual ingredients upon topical application.

3. What about using a product with L-ascorbic acid and then one with niacinamide ?

 I wouldn't worry much.

4. What about using a product with Vitamin C (derivatives and not L-ascorbic acid) and then one with niacinamide?

Again, wouldn't worry about that. Rather, they can easily be formulated together in the same product. 

Hope this post has answered most of your concerns.

The Formularx Vitamin C + Arbutin + Nag serum (which contains ethylated ascorbic acid) and the Barrier Relief moisturizer (which contains niacinamide) can be used together in the same routine if you are looking for brightening benefits.