What kind of substances should i avoid while i am taking phentermine?

Any other stimulants: Anything that stimulates your nervous system on top of what Phentermine already does can be dangerous. Avoid caffeine, attention deficit drugs, street drugs etc.

See below: Phentermine is used together with diet and exercise to treat obesity (overweight) in people with risk factors such as high blood pressure, high cholesterol, or diabetes. Do not use Phentermine if you are pregnant or breast-feeding a baby. You should not take Phentermine if you have a history of heart disease, severe or uncontrolled high blood pressure and maoinhibitors discuss with your doctor.