Halitosis is a word commonly used to refer to bad breath, and halitosis odors can come from a variety of sources. Most commonly, bad breath comes from the mouth, specifically from the back of the tongue where lots of bacteria are living, breaking down proteins found in saliva, dead cells from the body, and bits of food left there from your last meal. In the process of breaking down these proteins, the bacteria give off gases that contain sulfur molecules (often called volatile sulfur compounds), and these are the source of the offensive odor we frequently detect on the breath of others.

Classic bad breath from the mouth is probably the most offensive of halitosis odors, but there are other odors that can show up on the breath. Garlic is a well-known source of garlic breath, another manifestation of sulfur molecules that escapes not only on the breath but in the sweat as well. Onions, curry dishes, and strong cheeses can come back to you on your breath too. Smoker's breath is another common breath odor that persists on the exhalations and clothes of a smoker. These halitosis odors are obviously the result of personal habits, and if the habit persists, so will the bad breath.

Medical conditions are another source of halitosis odors, and these can range from "sweet smelling" to very offensive, depending on what is wrong. In the case of ketoacidosis, a dangerous complication of diabetes, breath is sweet smelling because of metabolic products that are being expelled in the breath. At the other end of the range, malignancy (cancer) of the upper respiratory passages can have a foul odor that is detected on the breath. Liver and kidney problems can also result in odd smells on the breath, so if a strange odor is noted and it can't be explained by oral bad breath or a recently eaten food, a visit to the doctor is in order.

Finally, drugs can cause peculiar halitosis odors ranging from chemical smells to odors that are less easily described. If you're are taking medications and you notice an unusual taste or odor, or if someone else remarks that there is a strange odor on your breath, check with your doctor or pharmacist to see if your medications might be the cause. Unless it is particularly offensive, this is one type of breath odor you can probably live with.