I tried Phentermine for a few days. It would take about an hour to kick in and didn't last very long. And when it did wear off I felt SO hungry that.. I'd eat something bad.
I wasn't very motivated then, though.
I say just do it the natural way. Though if you want to know about side affects of Fastin (It's the generic version of Phentermine, btw) here:
SIDE EFFECTS: Blurred vision, dizziness, dry mouth, sleeplessness, irritability, stomach upset or constipation may occur the first few days as your body adjusts to the medication. If these effects persist or become bothersome, inform your doctor. Notify your doctor if you experience: chest pain, nervousness, pounding heart, difficulty urinating, mood changes, breathing difficulties, swelling. If this medication makes you dizzy or lightheaded, avoid driving or engaging in activities requiring alertness. If you notice other effects not listed above, contact your doctor or pharmacist.
PRECAUTIONS: Tell your doctor your complete medical history, especially if you have: high blood pressure, an over-active thyroid, glaucoma, diabetes, emotional problems. This medication can be habit forming and must be used with caution. Alcohol can increase unwanted side effects of dizziness. Limit alcohol use. This drug is not recommended for use in children. Consult your doctor or pharmacist for further information. This medication should be used only when clearly needed during pregnancy. Discuss the risk and benefits with your doctor. This drug may be excreted into breast milk. You may have to stop nursing or stop using this drug. Consult your doctor before breast-feeding.