Eating, drinking and speaking all disturb the sensitive skin around the mouth meaning that healing can be particularly difficult.
The problem is that these lip splits are not a simple "sore", they are very likely to be caused by the common condition known as angular cheilitis.
This is caused by a bacterial or fungal infection which affects the damp, warm area around the mouth and is notoriously difficult to eradicate once it has taken hold.
The sore splitting lips caused by angular cheilitis can sometimes be eased by identifying and removing the triggers if possible.
For some, this can be caused by ill-fitting dentures.
In others it can be caused by a tendency to suck pens or fingers, causing tiny amounts of moisture to escape from the edges of the mouth.
Vitamin deficiencies can also be responsible.
Therefore, if appropriate, have dentures corrected and paint the offending items, such as pens and fingernails with bitter tasting harmless liquid, such as vinegar.
The key to treatment is to kill off the bacteria and fungal infection and the way to do this is to temporarily starve the affected area of oxygen and dry up the skin.
Once this is done, the bacteria and fungus cannot survive and healing will normally take place overnight.
Temporary treatment to ease sore splitting lips can be as simple as applying a thin layer pf petroleum jelly and protecting the area from extremes of temperature, as both heat and cold can cause significant pain.