It is only possible to find oral temperatures on patients who can hold the thermometer correctly in their mouth. So small children cannot use this method. It is also a problem for people with a cough or people who are vomiting. In the past it was a big problem because mercury thermometers needed a long time to measure the temperature. Today's digital thermometers are faster. If a person drinks something hot or cold, one still needs to wait before testing their oral temperature. 