What does vitamin C serum do?

She explains why the skincare industry is keen to use vitamin C:

Anti-ageing

"In its purest, most evidence-based form, L-ascorbic acid is an anti-oxidant - a substance that may prevent or delay some types of cell damage," says Mansberg. "This is probably why it's so good at reducing premature ageing. It's effective at protecting our skin from sun-induced skin changes that we see in the skin as we age. That also includes damage from sunburn, which vitamin C can help to prevent."

She adds that vitamin C also supports collagen production, which helps to reduce fine lines, wrinkles, and improve skin texture.

Healthy skin barrier and hydration

Vitamin C also helps to maintain a healthy skin barrier - the outermost layer of skin that protects against skin-damaging free radicals caused by external stressors like pollution. A healthy skin barrier also retains water, keeping your skin hydrated.

Hyperpigmentation

Mansberg explains that vitamin C can reduce the appearance of hyperpigmentation - patches of skin darker than the surrounding colour.

She says: "This is through a combination of its ability to switch off the main enzyme needed to make pigment in the skin, called tyrosinase, and its antioxidant and anti-inflammatory properties."

Your vitamin C product should:

• Contain L-ascorbic acid - the best active form of vitamin C for skin. Manufacturers often try and get around these issues by bonding vitamin C with other related chemicals, such as magnesium ascorbyl phosphate and tetrahexyldecyl ascorbate, but there is no evidence that they work.

• Have a pH lower than 3.5 - this refers to how acidic or alkaline a substance is. If a vitamin C serum doesn't disclose its pH that should be a red flag.

• Be in an airless pump bottle - not a glass dropper which exposes your vitamin C to air and light.

• Have minimal water content - all should contain water, but oil should be the first ingredient listed.

• Have a strength of 10% to 20%.

You can introduce it gradually into your skincare routine by:

1. Applying a small sample to your forearm.

2. Wait to see if your skin reacts badly.

3. If no side effects occur, apply it to your face as the label instructs.

4. To gradually increase your tolerance, you may wish to either start applying a smaller amount than instructed per day or apply just three times a week, for the first two weeks.