Wait for 20 to 30 minutes after smoking , eating, or drinking a hot or cold liquid. There is very little discomfort. There is essentially no risk. There is a rare risk of bowel perforation if the rectal thermometer is not carefully inserted. It may be helpful in monitoring to see if a treatment is working, especially in antibiotic treatment of infections.
Most fevers are a sign of infection and occur with other symptoms.
Abnormally high or low temperatures can be serious, and you should consult a health care provider.