Hyaluronic Acid rarely causes severe side effects or allergic reactions. However, people who have a history of severe allergic reactions should use caution and apply a patch test on the skin before regular use. Since hyaluronic acid is a humectant, it pulls moisture from the nearest source. If you're applying hyaluronic acid to a very dry face, it can actually end up drawing moisture from the deeper levels of your skin, which can leave your complexion feeling tight, uncomfortable - the opposite of the desired effects. This is why we recommend applying our serum only on damp skin and then follow with a moisturizer after 1-2 minutes to seal it in. In regards to the vitamin c element, we formulated our serum to have just enough vitamin C percentage to remain effect (5%) but low enough to ensure the least possible amount of irritation for all skin types. Negative effects could be irritation, redness or dryness. If irritation occurs, stop use and consult a doctor.