One might think that it's pretty obvious that if you give someone who has elevated blood glucose a drug designed to lower blood glucose, that the blood glucose level will probably go down. That's the logic behind all the diabetes medications -- they lower blood glucose in people with diabetes. But what if the person taking the diabetes drug doesn't have diabetes?
It's been well-recognized that some diabetes drugs will lower blood glucose (BG) in anyone taking them, even if the person's BG level is perfectly normal. That's true of the sulfonylurea (SU) drugs, and for insulin. If someone without diabetes accidentally (or deliberately) takes a SU drug or insulin, their blood glucose will fall below normal. Other diabetes drugs are generally thought to have little if any ability to lower blood glucose below normal in people who have normal blood glucose. Several classes of diabetes drugs, including metformin and the thiazolidinediones (Actos/pioglitazone and Avandia/rosiglitazone) are unlikely to cause low glucose levels in people with normal BG levels.
What about people with slightly-elevated blood glucose levels, that are higher than normal but not quite high enough to be diagnostic of diabetes? Folks with this situation are described as having "impaired glucose tolerance" (IGT) and are frequently called "prediabetic" because it's commonly believed that their blood glucose levels will eventually go up with the passage time, so that they eventually will have high enough BG levels to be labeled as having diabetes. Would drugs that work to lower BG in people with diabetes work to lower elevated BG in people with prediabetes? After all, the cut-point to distinguish prediabetes from "real" diabetes is arbitrary: It has been set as 126 mg/dl when fasting -- so if someone is consistently 126 or higher, they have diabetes; if consistently just below 126, they are prediabetic. It seems likely that whether someone is just over 126, or just under 126, that diabetes medications should work to lower BG. And if someone with prediabetes is given a glucose-lowering drug, and it lowers the BG, then one could say the drug has "prevented" diabetes. Or if you prefer, one could alternatively say that using the drug has masked the development of diabetes.
There are several studies showing that giving diabetes drugs to people with prediabetes do indeed lower BG levels (and "prevent" -- or mask, or delay -- the onset of diabetes).
In a study published almost a decade ago in 2002, called the Diabetes Prevention Program (DPP), researchers demonstrated that giving the diabetes drug, metformin, to patients with IGT would reduce the risk of developing diabetes, although diet and exercise worked even better.
The same year, another diabetes drug, acarbose, was also reported to prevent progression to diabetes (STOP-NIDDM Trial).
In a study published in 2006, another drug (Avandia/rosiglitazone) was shown to reduce the risk of developing type 2 diabetes by 62 percent compared to placebo (DREAM - Diabetes REduction Assessment with ramipril and rosiglitazone Medication).