Alli versus Phentermine

A Comparison of Alli and Phentermine

There are many weight loss drugs available on the market, including some over-the-counter medications like Alli.

As part of our overview of weight loss medicines, we will investigate the pros and cons of Alli and phentermine to help you decide if one of these drugs could aid your weight loss journey.

What Is Alli?

Alli is an over-the-counter weight loss drug that has been approved by the Food and Drug Administration (FDA). Alli can be purchased by adults who are overweight or obese, with a body mass index (BMI) of 25 or higher.

Alli is the brand name for 60mg of orlistat. At a higher dose of 120mg, orlistat is marketed as the prescription weight loss drug Xenical.

Orlistat may help you to lose weight by preventing the body from breaking down and absorbing the fat we consume in our diet. However, for best results and to avoid side effects, Alli should be taken while following a low-fat, low-calorie diet. (1) (2)

In trials, participants who took 60mg of orlistat for 24 weeks lost more weight than those who took a placebo.

Most people can expect an average weight loss of 5 to 10 pounds with six months of treatment. In most cases, Alli can still be taken if you have weight-related health complications, including high blood pressure. (3) (4)

What Is Phentermine?

FDA-approved phentermine tablets contain phentermine hydrochloride. This active ingredient boosts energy levels and reduces appetite, which may make it easier for some people to achieve a healthy weight.

You are likely to achieve greater weight loss if you take phentermine alongside a low-calorie diet and increased physical activity. Phentermine is a generic pill, but it is also prescribed as the brands Adipex-P and Lomaira. (5)

Phentermine stimulates the central nervous system. This leads to the release of neurotransmitters, including epinephrine.

The heart rate increases, extra energy becomes available, and the appetite is reduced. In clinical trials, 50% of participants taking 37.5mg of phentermine for around 3 months lost at least 10% of their baseline body weight. Improvements in cholesterol level and waist circumference were also noted. (6)

Phentermine vs. Alli: A Side-by-Side Comparison

You may be wondering, is Alli similar to phentermine? Although both pills aid weight loss, they work in very different ways. Our table makes the similarities and differences between these two drugs easier to understand.

Can You Take Phentermine and Alli Together?

Phentermine and Alli

Phentermine and Alli do not interact with each other, so in theory, they could be taken together. However, although an interaction has not been found, this does not mean that one doesn’t exist.

It is, therefore, wise to only take one weight loss drug at a time unless you are prescribed multiple medications under medical supervision. (15)

Phentermine or Alli – Which One Is Better for Me?

If you are wondering, ‘is Alli better than phentermine?’, the answer may depend on your lifestyle and past medical history. This is because the two drugs have different mechanisms of action and risk profiles.

Phentermine is a central nervous system stimulant that reduces the appetite, increases the heart rate, and boosts energy levels to help you lose weight. Alli reduces the amount of dietary fat that is absorbed. Both medications have been proven to aid weight loss when compared to a placebo in clinical trials.

Conveniently, at its full dose phentermine only needs to be taken once a day. Conversely, Alli is taken three times daily with each meal that contains fat.

Care must be taken to eat a low-fat diet when taking Alli, as excess fat consumption can lead to unpleasant side effects, including difficulty controlling bowel movements.

If Alli aids in weight loss, it can be taken for up to six months. After this, medical advice is recommended regarding the continuation of treatment. Phentermine is only FDA-approved for 12 weeks, although some practitioners will support concurrent 12-week treatment cycles if significant weight loss is seen.

Alli is more expensive than generic Phentermine, but your health insurance may cover the cost of either drug.

Both medications have side effects, and there is a possibility of adverse events, whichever drug you choose. In general, Alli has a lower risk profile and fewer contraindications to its use. To be sure that you choose the right medication for your individual circumstances, you should seek medical advice prior to commencing treatment.

Make sure to also read our analysis of the other weight loss drugs currently available in the U.S. market.


1. FDA (Orlistat).
2. EL, Thomas et al. Pragmatic study of orlistat 60mg on abdominal obesity. European Journal of Clinical Nutrition. 2011; 65: 1256-1262.
3. TJ, Smith et al. Case Study: Efficacy of Orlistat 60mg Treatment for Weight Loss.Topics in Clinical Nutrition. 2012; 27: 339-347.
4. FDA (Alli Label).
5. (Phentermine).
6. Kim, KK et al. Effects on weight reduction and safety of short-term phentermine administration in Korean obese people. Yonsei Medical Journal. 2006; 47: 614-625.
7. (Alli FAQs).
8. (Phentermine Dosage).
9. (Phentermine Side Effects).
10. (What is Phentermine?).
11. (Buy Generic Phentermine).
12. (Buy Alli).
13. (Buy Alli).
14. (Generic Alli).
15. (Phentermine Interactions).