WHAT IS A SERUM?
A serum is a concentrated blend of targeted actives in a very light textured fluid. Unlike a moisturiser they do not offer proper hydration or lipid protection (you need oils for that) and are usually designed to target particular concerns.
DO I STILL NEED A MOISTURISER?
Yes. Even though most offer some form of hydration you will still need to use a moisturiser, though oily skin types can get away with just using a serum in the summer. Hyaluronic Acid serums are very popular as they are believed to hydrate the skin. In actual fact, they can often dehydrate the skin especially when used in very arid and dry weather conditions, as the water molecules at HA attract are most likely to be those at are already in your skin. To get around this, always use an HA serum on damp skin.
WHEN AND HOW TO I USE A SERUM?
Unless a strong retinol or acid serum (which should be used once at night), to get the most out of your serum, always use twice a day, morning and night, directly onto freshly cleansed skin. If you use a derma roller, use straight after as it will boost penetration. The rule of thumb is to always start with water-based to oil-based, and lighter texture to heavier texture. So, cleanse, tone, serum, moisturiser then oil.
CAN I MIX SERUMS TOGETHER?
Unless the serums are specifically formulated to be mixed, I would be wary of mixing serums together. Many many ingredients are very volatile and mixing can alter perfectly-formulated pH stable formulations which can have a huge effect on ingredients and efficacy. Formulations can take a very long time to get right, so you don’t want to fiddle too much. Never mix acids with other products, unless you want to buffer the effect of the said acid. The wonderful thing about serums though is that they are very lightweight, so you can use more than one at a time - three or four if you wish - just always allow each to fully absorb before applying the next. This way you will get the most out of each one. If you have the time leaving a few minutes between each will be very beneficial.