Phentermine

Phentermine is a weight-loss medication that is available by prescription only. It works by suppressing the appetite and possibly increasing the metabolism. Phentermine, which is approved for short-term use only, comes in the form of tablets and capsules. Potential side effects of the drug include dizziness, insomnia, and restlessness. Phentermine is a controlled substance, meaning it has the potential to be abused.

 

What Is Phentermine?

Phentermine is a prescription weight-loss medication approved for short-term use only.
 
(Click Phentermine Uses for more information on what phentermine is used for, including possible off-label uses.)
 

Phentermine Products

The differences between all the various phentermine products can be confusing. Phentermine was sold under numerous different brand names in the past (Fastin®, Ionamin®, Obestin-30®, Oby-Trim®, Ona-Mast®, Pre-Sate®, and Tora®), but these brand-name products are no longer available. One brand-name product is still available (Adipex-P®). Generic phentermine 30 mg capsules come in two different colors. Blue capsules (with little beads) are generic for Fastin, while yellow capsules (with powder instead of beads) are generic for other phentermine products. There is no generic available for Ionamin, a long-acting form of phentermine that is no longer available.
 

Who Makes Phentermine?

Phentermine is currently produced by numerous different manufacturers of generic medications.
 

How Does Phentermine Work?

Phentermine works by suppressing the appetite, and it may also increase the metabolism. It is chemically related to amphetamines.
 

Phentermine Effects

Only very short studies have evaluated the effectiveness of medications like phentermine. In these studies, people taking these medications lost more weight than those not taking the medication, but the difference was very small (only a fraction of a pound more per week for the people taking the weight-loss medication). Studies also suggest that the effects of these medications for weight loss decrease after the first few weeks.
 

When and How to Take Phentermine

General considerations for when and how to take phentermine include the following:
 
  • Phentermine comes in tablet and capsule form. It is usually taken by mouth once daily, although some people may need to take it twice a day.
     
  • Phentermine should be taken either before breakfast or one to two hours after breakfast, depending on the particular phentermine product.
     
  • Phentermine should be combined with an appropriate diet. It is not meant to be used without dieting.
     
  • Do not take phentermine more often (or at higher doses) than prescribed, as this can lead to phentermine addiction or dependence.
     
  • For the medication to work properly, it must be taken as prescribed.
     

Phentermine Dosing Information

The dose of phentermine that your healthcare provider recommends will vary depending on a number of factors, including:
 
  • The particular phentermine product
  • How you respond to phentermine
  • If you develop bothersome or dangerous side effects of phentermine
  • Other medical conditions you may have
  • Other medications you may currently be taking.
     
As is always the case, do not adjust your phentermine dose unless your healthcare provider specifically instructs you to do so.
 
(Click Phentermine Dosage for more information.)
 

Side Effects of Phentermine

As with any medicine, side effects are possible with phentermine. However, not everyone who takes the drug will experience side effects. In fact, most people tolerate it quite well. If side effects do occur, in most cases, they are minor and either require no treatment or can easily be treated by you or your healthcare provider. Serious side effects are less common.
 
Common side effects of phentermine include, but are not limited to:
 
(Click Phentermine Side Effects to learn more, including potentially serious side effects that you should report immediately to your healthcare provider.)
 

Drug Interactions With Phentermine

Phentermine can interact with a number of other medications (see Phentermine Drug Interactions).
 

Phentermine: What Should I Tell My Healthcare Provider?

You should talk with your healthcare provider prior to taking phentermine if you have:
 
Also, let your healthcare provider know if you are:
 
Make sure to tell your healthcare provider about all other medicines you are taking, including prescription and prescription medicines, vitamins, and herbal supplements.
 
(Click Phentermine Warnings and Precautions to learn more, including information on who should not take the drug.)
 

What If I Take a Phentermine Overdose?

People who take too much phentermine may experience the following symptoms:
 
If you happen to overdose on phentermine, seek medical attention immediately.
 
(Click Phentermine Overdose for more information.)
 

Phentermine: Storage Methods

Phentermine tablets and capsules should be stored at room temperature, away from moisture and heat. Keep phentermine tablets and capsules in an airtight container.
 
Keep phentermine and all other medications out of the reach of children.
 

What Should I Do If I Miss a Dose of Phentermine?

If you do not take your phentermine as scheduled, take your missed dose as soon as you remember. If it is almost time for your next dose, skip the missed dose and continue with your regular schedule. Do not take a double dose of phentermine.
 

Phentermine Strengths

Phentermine comes in the following strengths and forms:
 
  • Phentermine 15 mg capsules
  • Phentermine 30 mg capsules (blue color), equivalent to Fastin
  • Phentermine 30 mg capsules (yellow color), not equivalent to Fastin
  • Phentermine 37.5 mg capsules
  • Phentermine 37.5 mg tablets.
     

Is There a Generic Version of Phentermine?

Since most of the brand-name phentermine products have been discontinued by their manufacturers, most phentermine products available (except Adipex-P) are generic (see Generic Phentermine).
 

Is Phentermine Available Without a Prescription?

Not only is phentermine a prescription-only medication, but it is also a controlled substance. This means that it is a drug that has the potential to be abused and is regulated by strict laws. It is illegal to sell or obtain phentermine without a legitimate prescription (see Phentermine Without a Prescription for more information).
 
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Written by/reviewed by: Kristi Monson, PharmD; Arthur Schoenstadt, MD
Last reviewed by: Kristi Monson, PharmD;