Short answer: Xenical (orlistat) is a 120 mg capsule taken with each meal that contains fat, approved for weight loss and keeping it off.1 Phentermine, here from the Lomaira 8 mg label, is an appetite suppressant for short-term use of a few weeks, and its label gives it as Schedule IV.2 The Lomaira label advises against combining it with other weight-loss drugs. Neither label reports a trial against the other.
| Xenical (orlistat) | Phentermine (Lomaira) | |
|---|---|---|
| How it works | Blocks some fat in a meal from being absorbed1 | Appetite suppressant, chemically related to amphetamine2 |
| Approved for | Weight loss and keeping it off, and less regain after weight loss; BMI 30 or more, or 27 or more with risk factors1 | Short-term add-on to a weight-loss plan; BMI 30 or more, or 27 or more with risk factors2 |
| Length of use | Weight loss and maintenance; label studies ran up to 4 years1 | A few weeks2 |
| Label dose | 120 mg three times a day with each main meal containing fat1 | 8 mg three times a day, half an hour before meals; late evening doses avoided2 |
| Weight loss in label trials | Pooled 1-year trials: 13.4 lbs vs 5.8 lbs on placebo1 | Appetite suppressants added only a fraction of a pound a week over placebo in short trials2 |
| Controlled substance | No schedule listed1 | Schedule IV2 |
| Common side effects | Oily spotting 26.6% vs 1.3% on placebo; gas with discharge 23.9% vs 1.4%1 | Listed without rates: fast heartbeat, raised blood pressure, insomnia, dry mouth, restlessness2 |
| Missed or skipped dose | If a meal is skipped or has no fat, that dose can be left out1 | No rule in the label2 |
| Boxed warning | None1 | None2 |
Xenical works in the gut: it stops some of the fat in a meal from being absorbed, so its side effects are mostly oily spotting, gas with discharge and urgent bowel movements, which usually began within 3 months. A daily multivitamin is taken at least 2 hours apart from it.1 Phentermine reduces appetite. The Lomaira label says the extra weight loss over diet alone in short trials was clinically limited, and that tolerance has been seen with all drugs of this class; if it develops, the drug is stopped rather than the dose raised.2
Xenical's warnings include rare severe liver injury, oxalate kidney stones and kidney damage, gallstones and lower absorption of vitamins A, D, E and K; it is not used in pregnancy.1 Lomaira's include rare primary pulmonary hypertension and heart valve disease, abuse and dependence, raised blood pressure and an adverse reaction with alcohol; it is not used with heart or blood vessel disease, an overactive thyroid, glaucoma, a history of drug abuse, or within 14 days of an MAOI.2 These numbers come from separate trials with different people, lengths and methods, so they are not a head-to-head comparison.
Which, if either, fits is for your prescriber. More detail: Xenical side effects, Lomaira side effects, the Xenical and Lomaira dosing schedules, the Xenical missed-dose page, the orlistat and phentermine guides, and Xenical vs Qsymia: how do the two weight-loss pills differ?.