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Amphetamines can keep exhaustion at bay, according to a new study.

Researchers administered amphetamines on exercising male rats and found that while drugs delay time to exhaustion, it can also increase the risk of suffering from exertional heat stroke.

Researchers said that the increase time to exhaustion is dangerous because fatigue helps the body know when to stop. Exhaustion occurs when the body's core temperature reaches a potentially dangerous point, and the delay caused by amphetamines can increase the risk of exertional heat stroke.

The latest study called "Amphetamine increases Vo2max and time to exhaustion at the expense of body temperature and economy" will be presented at 2 p.m. on Wednesday during the heat stress session at the American College of Sports Medicine annual meeting in Orlando, Florida.

Scientists predicted that amphetamines boost the time to exhaustion by allowing the core temperature to climb higher before exhaustion occurs. Rusyniak and his team also wanted to determine whether oxygen consumption was affected and if it played a role in the threshold for exhaustion.

In the study, rats practiced running on the treadmill for several days before the experiment. During the experiment, the room was set at 89.6 degrees Fahrenheit. Rats in a control group were given saline injections, and two other groups were given amphetamine injections of either 1 milligram or 2 milligrams per kilogram of body weight.

The findings revealed that animals receiving amphetamine had significantly higher core temperatures when they reached exhaustion. Surprisingly, researchers found that animals receiving the higher dose of the amphetamine got to this critical temperature faster; therefore their time to exhaustion was not increased.

The findings also show that muscles used up oxygen at a higher rate in rats receiving the larger amphetamine dosage. However, there was no evidence that the efficiency or "economy" of the oxygen use was improved with amphetamines.

"The benefit of amphetamine use was probably not related to anything other than raising the temperature at which exhaustion occurs, which from a medical safety standpoint is not a benefit at all," Rusyniak said.