All that festive puckering-up can leave the lips feeling exhausted at this time of year. The muscles that take most of the strain are orbicularis oris (which makes a sphincter of the mouth) and buccinator (which presses lips and cheeks against the teeth). The lips also function as a portal for food, in the articulation of speech and facial expression.

The distinct line that separates the red part of the lip from its paler surroundings is called the vermilion border; thin skin and a profusion of capillaries give it its distinctive colour. The groove between upper lip and nose is the philtrum, and the creases that run from the corners of the mouth up to the sides of the nose are the nasolabial folds: these deepen with age and are one of the commonest sites for injecting facial fillers. It has been suggested that the fuller a woman's lips, the higher her oestrogen levels, a hormone associated with fertility.

Cleft lip and/or palate is the commonest facial birth defect in the UK, affecting one in 700 babies and correctable by surgery. Squamous cell carcinoma is the commonest cancer on the lip and usually appears as a non-healing spot or lump. Risk factors include tobacco, alcohol, sun and human papilloma virus. Treatment may include surgery, radiotherapy and chemotherapy.

A bluish tinge to the lips may suggest cyanosis, an abnormally low oxygen concentration in the blood. Cold sores are the result of herpes simplex type one. Initial infection is through contact with someone with the virus. Once contracted, herpes simplex lies dormant in a part of the nervous system known as the dorsal root ganglion, and may be reactivated many times in a person's life when they are under par.