When a patient presents at my office with pain attributable to a cavity, I sometimes place a temporary filling in the tooth and reappoint the patient for a final permanent filling at another visit. Sometimes, this is done in order to save time, especially if we have slipped the emergency patient between two regularly scheduled patients. Sometimes it is done in order to save money.
Temporaries are the least expensive (and most temporary) way to fill a tooth. Temporary fillings can be done quickly, because they are usually inserted without any of the time consuming rituals associated with a permanent filling. The patient is anesthetized, the decay removed and the temporary filling is mixed and inserted, generally simply by pushing it into the cavity preparation with a gloved finger. The patient bites into it while it is still soft in order to adjust the height, and the patient leaves the office without even waiting for a final set on the material. In a phrase, a temporary is "fast and cheap'.
But there is another reason that may indicate that a temporary is the best way to treat the patient, even if time or money is not an issue. Temporary fillings are different from permanent amalgam or composite fillings because they are "sedative" fillings. This means that they tend to soothe an inflamed nerve in a tooth, and may make the difference between the tooth needing a root canal (or an extraction), or simply filling the tooth later on, after the nerve has calmed down. Sometimes a temporary filling is the best course to relieve pain.
Temporary fillings are made of two major components: Oil of clove (eugenol), which has been used for centuries to relieve toothaches, and Zinc Oxide which is the ingredient that makes Desitin diaper rash ointment white. Zinc oxide is an excellent disinfectant. The oil and oxide mix together to make a stiff paste that eventually hardens into a waterproof substance which soothes the nerve of the tooth and kills germs while protecting the cavity like a hard band aid. When used as a temporary filling material or cement, this material is called "zinc oxide and eugenol", or ZOE for short.
Zinc Oxide and Eugenol (ZOE) is not very durable, and it wears away after just a few weeks, but it works to relieve pain, calm the nerve and protect the tooth until an appointment can be made to get it filled permanently. During the Vietnam war, the US Army invented a more durable form of ZOE called Intermediate Restorative Material (IRM) which is fortified with plastic powder. (It originally came in red, white or blue colors.) IRM is used almost universally in dental offices throughout the world for temporary fillings. The increase in durability allows the temporary to last three to 6 months (sometimes even longer).
Never plan to keep a temporary filling more than 6 months. They are not meant to last that long, and while the eugenol lulls the patient into a false sense of security, the restoration wears rapidly and begins to leak. If you wait too long, the nerve could die, the temporary filling will wear away, the tooth will decay further, and then you will need a root canal or extraction.