Fundamentally Alli is a lower dose version of Xenical. However, in contrast to Xenical, Alli is available over the counter without a medical prescription. Even though Alli is available without a doctor prescription, if you are considering taking Phentermine in combination with Alli you should discuss it with your physician first.
Phentermine is an appetite suppressant. Alli inhibits fat absorption into the body. So depending on your personal medical history and condition your doctor may allow both medications to be used in conjunction if you are seriously obese.
Your doctor will want to know your medical history and any medications you are currently taking, to determine if you may safely take Phentermine and Alli together.