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Drugs used for weight loss are called anorexiants or diet pills. Obesity is a complex disorder and often requires long-term treatment to promote and sustain weight loss.

Therapies and drugs (prescription, over-the-counter supplements, herbs, medicines, etc.) should be taken only under the supervision of a qualified physician, and as part of an overall weight loss program that includes diet, exercise, and psychological and lifestyle supportive counseling.

Using prescription weight loss drugs to treat obesity should be used as an option for the following individuals:
  1. People with a body mass index of 30 and above with no obesity-related conditions.
  2. A person with a BMI of 27 and above with obesity-related conditions.

Types of prescription diet pills

Different diet pills work in different ways depending on which ingredients they contain.

Appetite suppressants

Most available weight loss medications are "appetite suppressants" medications. Appetite suppressants affect the appetite-regulating region of the brain called the hypothalamus. They work by blocking the re-uptake of the chemicals serotonin and norepinephrine, which create the feeling of satiety that develops after eating a big meal. With more of these chemicals circulating in brain, there is a feeling of fullness and lesser urge to eat.

The active ingredient in appetite suppressants most is phentermine hydrochloride or phendimetrazine, which are sympathomimetic amines. These chemicals affect appetite control centers in the brain, causing you to eat less. Phentermine-based diet pills include the generic form of the drug, as well as Meridia, Bontril, Ionamin, and Adipex. Another appetite suppressant is Didrex, which uses the active ingredient benzphetamine hydrochloride as an anorectic agent.

Tenuate uses a different active ingredient, diethylpropion. Diethylpropion is a sympathomimetic amine that stimulates the central nervous system, increasing the heart rate and blood pressure while decreasing appetite, which promotes weight loss.

Prescription fat blockers

Prescription fat blockers, such as orlistat (Xenical), affect the digestive system by inhibiting the action of an enzyme called lipase. Lipase normally breaks down the fat in food when it gets to the intestinal tract. With a lipase inhibitor in effect, a percentage of the fat ingested is removed from the body through bowel movements instead of being broken down and absorbed. Xenical is intended for individuals who are considerably overweight or obese.

Many prescription drugs are used as diet aids even though that was not their intended purpose. Some antidepressants are used "off-label" as diet aids because they have been shown in studies to help patients lose weight and keep it off for several months. Researchers are also studying certain drugs normally used to treat epilepsy and diabetes for their weight loss potential.       
Currently, most available weight loss medications approved by the FDA are for short-term use, meaning a few weeks or months. Meridia and Xenical are the only weight loss drugs approved for longer-term use in significantly obese people, although the safety and effectiveness have not been established for use beyond 2 years.

Do prescription diet pills really work?

The effectiveness of diet pills and their main ingredients have been extensively studied by many research institutes and organizations worldwide. Xenical and Meridia are moderately effective, with an average weight loss of 5 to 22 pounds or up to 10% of the body weight over a 1-year period, more than what would be expected with non-drug treatments. However, the response to these medications is based on each individual, and some people experience more weight loss than others. 

There is no one correct dose for these medications. Your doctor will decide what works best for you based on his or her evaluation of your medical condition and your response to treatment. Patients generally experience a maximum weight loss within six months of starting medication treatment.

Over the short term, weight loss from prescription drugs may reduce a number of health risks in obese people. There are currently no studies to determine and prove the effects of these medications over the long term.

Possible risks and side-effects of prescription diet pills

When considering long-term weight loss drugs for obesity, the following possible concerns and risks should be discussed with your doctor:


Currently, all prescription medications to treat obesity except Xenical are "controlled substances." So, doctors are required to follow certain restrictions when prescribing them since they could be addictive. Although abuse and dependence are not common with non-amphetamine appetite-suppressant medications, doctors should be cautious when they prescribe these medications for patients with a history of alcohol or other drug abuse.

Developed tolerance

Most people's weight tends to level off after 6 months while taking a weight loss medication. This leads to a concern that body might have developed a tolerance to these drugs' effects. After that, if you don't also follow a healthy eating and exercise plan, the weight could come right back. However, it is unclear whether this leveling off is indeed due to a developed tolerance or if the medication has reached its limit in effectiveness.


There are many side-effects of prescription weight loss drugs that you must be aware of before you begin taking them. Knowing the side-effects of prescription weight loss drugs can help you to avoid choosing the wrong weight loss drug for your body. Various unpleasant side-effects associated with these drugs include:
  • Increased heart rate
  • Increased blood pressure
  • Sweating
  • Constipation
  • Insomnia
  • Excessive thirst
  • Lightheadedness
  • Drowsiness
  • Stuffy nose
  • Headache
  • Anxiety
  • Dry mouth


Xenical  (Orlistat)  

Abdominal cramping, passing gas, leakage of oily stool, increased number of bowel movements and the inability to control bowel movements. People taking Xenical should take a multivitamin at least 2 hours before or after taking the medication because Xenical reduces the absorption of some vitamins. Meridia (Sibutramine)  
Increased blood pressure, dry mouth, headache, insomnia, and constipation. Meridia is not recommended for people with poorly controlled high blood pressure, heart disease, irregular heartbeat, or a history of stroke.

Adipex-P (Phentermine) -  Nervousness, sleeplessness, mood changes.

Discussing prescription weight loss drugs with doctor

Your physician will look at a number of factors to determine if you are a good candidate for prescription weight loss medication. He or she will determine how overweight you are and where your body fat is distributed. The doctor will carefully evaluate the potential benefits of taking a drug and weigh them against the possible long-term risks. The doctor will also consider your health history, the possible side-effects and the potential interaction of weight loss drugs with other medications being taken.

The doctor will ask you about any existing allergies that you may have, and whether or not you are pregnant or breastfeeding. Existing medical conditions may also affect the use of these drugs. It is imperative to inform the doctor in case you are suffering from any of the conditions including diabetes, high blood pressure, heart disease, epilepsy, kidney disease, glaucoma, alcohol or drug abuse, overactive thyroid, depression or other mental illness, migraine headaches requiring medication, planning for a surgery requiring general anesthesia.

People who are prescribed weight loss pills should follow the prescription carefully. Because appetite suppressants may cause drowsiness or lightheadedness, it is important to know how you respond to these medications before you attempt to drive or operate machinery.

Keep in mind that some health insurance plans don't cover weight loss prescription drugs. So, contact your health insurance provider if you're unsure of your prescription coverage.

Prescription medications – the real effectiveness

The weight loss pills are not a cure-all for obesity. As you consider weight loss drugs, make sure that you make every effort to exercise, change your eating habits and adjust any other lifestyle factors that have contributed to your excess weight. Weight loss drugs aren't the easy answer to weight loss, but they can be a useful tool to help you make the necessary diet and lifestyle changes.