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Updated March 25, 2015.

The alli weight loss aid is the only over-the-counter diet pill approved by the Food and Drug Administration to lose weight. The product is widely available at many pharmacies and markets.  Since alli doesn’t require a prescription, it’s easy to buy.  But the weight loss pill does have some side effects. So before you invest, you probably want to know if alli works to lose weight and if the diet pill is safe for you.

How Does alli^® Work?

alli is the brand name for a medication called orlistat.  If that drug name sounds familiar, you may have seen it associated with a prescription weight loss pill called Xenical.  alli contains 60 mg of the medication orlistat and does not require a prescription. Xenical contains 120 mg of orlistat and requires a prescription from your health care provider.

alli (orlistat) is a "lipase inhibitor." This means that it works by preventing your body from absorbing some of the fat that is in your food.  You take the weight loss pill before you eat each meal and it works in your intestines to remove about 25% of the fat that you consumed.

So where does the fat go? Excess fat is eliminated from your body in your stool. But that doesn’t mean you can eat fatty foods when you take alli.  The medication should only be used while you’re eating a balanced, low-fat diet.

alli^® Side Effects

If you’re considering taking alli to lose weight, you need to commit to a low calorie, low fat diet.

  That means that you’ll eat less than 30% of your total daily calories from fat and you won’t eat any single meal that contains more than 15 grams of fat.  Many alli users who complain about side effects experience them in response to eating a high fat meal.

But even with a careful diet, some alli users can have problems. According to the National Institutes of Health, some of the most common side effects of alli involve changes to your bowel movements.  Examples include oily or loose stool, an urgent need to go to the bathroom, oily spotting on your underwear, gas, stomach pain and difficulty controlling your bowel movements.  Side effects are generally experienced within the first few weeks of taking the pill.  If alli side effects continue beyond that point, you should talk to your doctor.

You should also make sure that you buy alli from a reputable vendor and check the package closely before you take the medication. In 2014 there were episodes of product tampering that resulted in a voluntary recall of the diet pills.  Now, alli is back on the market with an updated package that makes it easier to see if the bottle has been opened.

Is alli^® Effective?

Considering that the side effects may be uncomfortable or embarrassing, you probably want to know how much weight you can use when you take the medication.  In short, does alli work?

The answer depends on a number of factors, including your commitment to a low calorie diet and exercise program.  alli can increase the amount of weight you lose, but if you don’t stick to the eating plan, you probably won’t lose weight and if you don’t exercise on a regular basis, the weight probably won’t stay off for good.

When researchers studied weight loss with orlistat, they’ve seen mixed results. For example, in one study cited by GlaxoSmithKline (the company that makes alli,) the  medication helped 26 patients lose 5.6 percent of their total body weight in three months. That sounds great. But in another published study researchers followed over 100,000 orlistat users for three years.  They found that although patients lost about 2 pounds per month in the first few months, they did not maintain significant weight loss over the long-term.

Should You Take alli^®?

Just because alli is available to everyone, doesn’t mean that it is a good weight loss treatment for everyone.  According to the FDA, alli is approved for use in overweight adults along with a reduced-calorie, low-fat diet, and exercise program. The diet pill is not an easy fix for weight loss, but it may help some people lose more weight and improve their health.

If you are thinking about taking alli for weight loss, talk to your doctor before you take the medication.  A prescription is not required, but there are certain patients who should not take orlistat, including pregnant patients, people with cholestasis, malabsorption syndrome, anorexia nervosa, bulimia, diabetes, kidney stones, pancreatitis, or gallbladder or thyroid disease.  Your doctor is the best person to determine if the product is safe for you and how losing weight might improve your health.