Phentermine With Diazepam Interaction

There is no drug interaction between phentermine and diazepam.


I took a 30mg phentermine at 7AM yesterday morning. Today I was prescribed diazepam 5mg to be taken before bed. Is it safe to take the diazepam tonight as I haven’t taken phentermine since yesterday morning? How long does phentermine stay in the body?

Asked by Jay On Nov 28, 2018

Answered by
Medical Content Reviewed By PharmacistAnswers Staff

On Nov 29, 2018


There is no drug interaction between phentermine and diazepam and they can safely be taken together.

Having said that, they do have conflicting and potentially off-setting effects as phentermine is a stimulant while diazepam is a CNS (central nervous system) depressant.

Therefore, for an example regarding this issue, if you take diazepam for anxiety, you may find that it doesn't work quite as well if you take it with phentermine.

In terms of how long phentermine lasts, the duration of action differs depending on the specific product:

  • The duration of action of the 8 mg capsules or tablets is about 4 hours.
  • The duration of action of the 30 mg capsules or the 37.5 mg tablets is 12—14 hours.

Now, although it may not still be having active effects, it takes significantly longer than 12-14 hours for phentermine to be metabolized and eliminated from the body.

According to the prescribing information for phentermine, it has a half-life (time it takes for our body to metabolize 50% of a drug) of 19-24 hours.

Generally, it takes at least 5 half-lives need to elapse to consider a drug completely metabolized and eliminated from the body. This would equate to about 5 days with phentermine.

This is why before surgery, if you are taking phentermine, it is recommended to stop taking it at least one week prior, to avoid interactions with anesthesia and potential adverse cardiac effects.

In your situation, having taken phentermine more than 24 hours ago, the drug is still in your body, but should be having minimal effects on you. There is no interaction with your prescribed diazepam so you can feel safe taking it.

Section Summary

There is no drug interaction between phentermine and diazepam, although they do have conflicting effects (phentermine is a stimulant while diazepam is a central nervous system depressant).

About Phentermine

Phentermine is classified as a sympathomimetic stimulant medication, chemically similar to amphetamines. It is most often used for the treatment of obesity.

After taking by mouth, the effects from phentermine occur in as little as 30 minutes and can last from 4 to 14 hours, depending on the individual.

Due to the relative long acting effects of the drug, it should ideally be dosed in the morning to prevent issues with insomnia. The most commonly reported side effects of phentermine include:

  • Headache
  • Insomnia
  • Over-stimulation
  • Restlessness
  • Tremor

About Diazepam

Diazepam (Valium) is a benzodiazepine used for a variety of indications including:

  • Anxiety disorders
  • Seizures
  • Acute alcohol withdrawal

It is considered to be a 'long-acting' benzodiazepine, with a relatively long half-life of 30-100 hours.

It may be taken with or without food but if it upsets your stomach, take it with food or milk.

Benzodiazepines can cause withdrawal symptoms if you have been taking them regularly and it is important to not suddenly stop taking them. You must gradually reduce the dose or you may get severe side effects. 

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