Didrex and Phentermine

A Comparison of Didrex and Phentermine

The choice of weight loss prescription medications can be overwhelming.

As part of our series of drug comparison articles, we’ll compare Didrex to phentermine to assist you in exploring whether either of these prescription drugs might be right for you.

What Is Didrex?

Didrex belongs to a group of medications known as anorectics or appetite suppressants. The pills contain the active ingredient benzphetamine, a substance that shares some similarities with amphetamine.

It is prescribed for adults who have a body mass index (BMI) of 30 or higher and who have been unsuccessful in losing weight through diet and exercise alone. (1)

Didrex works by reducing feelings of hunger (appetite suppression) and stimulating the central nervous system. The metabolic rate is increased, with the release of norepinephrine occurring, too. This may make it easier for some people to lose weight. (1) (2)

The dosing schedule of Didrex depends on how effective the pills are. It is common to start taking a mid-morning dose of 25mg to 50mg, which may be increased as needed to a maximum dosage of 50mg three times daily.

In a clinical trial, weight loss of around one pound per week was noted in those taking Didrex. The FDA has approved Didrex but only for short-term use of up to 12 weeks. (3) (4) (5)

What Is Phentermine?

Phentermine also stimulates the central nervous system. Like Didrex, taking phentermine reduces the appetite, boosts energy levels, and increases the heart rate. Phentermine stimulates the release of the neurotransmitter epinephrine.

Although both weight loss prescription medications have similarities to amphetamine, the difference between Didrex and phentermine is that phentermine contains phentermine hydrochloride rather than benzphetamine. (6)

Phentermine can be prescribed as Adipex-P or Lomaira, but may also be obtained as a generic pill. In clinical trials, taking the highest dose of phentermine, 37.5mg daily, for 3 months led to around 10% weight loss in half of the participants.

The researchers also noted that weight loss was associated with improvements in cholesterol and a reduction in weight circumference. (7)

Like Didrex, phentermine is only approved by the FDA for short-term use of up to 12 weeks. (5)

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

Although both prescription drugs are appetite suppressants, due to contraindications or potential drug interactions, you may be better suited to phentermine or Didrex. Our phentermine vs. Didrex table gives a clear comparison of the similarities, differences, pros, and cons of each drug.

Is Didrex Stronger Than Phentermine?

As both drugs are central nervous system stimulants, many people wonder which is stronger, Didrex or phentermine?

In clinical trials, Didrex was found to support weight loss of around 1 pound per week. As the drug is only approved for 12 weeks, you could expect to see a maximum weight loss of 12 pounds (5.44kg). (3)

In a clinical trial, phentermine led to a weight loss of around 7.2kg in 14 weeks. (7) Recurrent 12-week cycles of phentermine are sometimes prescribed for those for whom phentermine supports successful weight loss.

As phentermine may cause greater initial weight loss and can be taken for longer, it may be considered the more effective drug option.

Can I Take Didrex and Phentermine Together?

Take pills Didrex Phentermine

Didrex and phentermine must not be taken together. You must wait for one year after taking Didrex before using any other weight loss drug.

Phentermine or Didrex – Which One Is Better for Me?

When weighing up the pros and cons of Didrex vs. phentermine, you may notice similarities as both diet pills are central nervous system stimulants.

As it is common to want to achieve maximum weight loss results, many people ask whether phentermine or Didrex is stronger. Although it is hard to say which of the two appetite suppressants is stronger, phentermine may offer a greater chance of success.

Phentermine is the most widely prescribed weight-loss medication in the USA. Some healthcare professionals will prescribe concurrent 12-week cycles of phentermine to patients who successfully lose weight. Phentermine may therefore lead to greater weight loss long-term compared to a short period of treatment with Didrex.

Generic phentermine pills are cheaper than generic Didrex tablets.

Both diet pills have a similar range of side effects, but, unlike Didrex, different-strength phentermine tablets are available so that a lower dose can be taken if side effects are experienced.

Phentermine is more commonly chosen than Didrex by those wishing to lose weight. To help you select the right drug for your personal circumstances, you should consult a healthcare professional for further advice.

Make sure to consult as well our analysis of the other prescription appetite suppressants currently available in the United States.


1. US Food and Drug Administration (Didrex).
2. RA, Plodkowski. Obesity Management: Clinical Review and Update of the Pharmacologic Treatment Options.Federal Practitioner. 2016; 33: 6-16.
3. Rxlist.com (Didrex).
4. Simkin B, Wallace L. A Controlled Comparison of Benzphetamine and D-Amphetamine in the Management of Obesity.The American Journal of Clinical Nutrition. 1961; 9: 632-637.
5. NIDDK (Didrex Approval).
6. Phentermine.com (Phentermine).
7. Kim, KK. Effects on weight reduction and safety of short-term phentermine administration in Korean obese people. Yonsei Medical Journal. 2006; 47: 614-625.
8. Phentermine.com (Phentermine Dosage).
9. Phentermine.com (Phentermine Side Effects).
10. Phentermine.com (Phentermine Warnings).
11. BlinkHealth.com (Buy Generic Phentermine).
12. BlinkHealth.com (Buy Generic Didrex).