Taking Benadryl With Phentermine

In our latest question and answer, the pharmacist discusses whether or not there is an interaction between Benadryl (diphenhydramine) and phentermine.

Mar 26, 2018

Cortana asked

Can you safely take Benadryl (diphenhydramine) and phentermine together?


Benadryl and phentermine (brand name Adipex-P) can be taken together in many cases, but there is considered to be a minor interaction between the two medications.

Benadryl and phentermine have opposing effects on the body and the respective effects of each medication may counteract each other. For example:

  • Benadryl has significant sedative properties.
  • Phenetermine, an amphetamine like medication, is a 'sympathomimetic' and stimulant.

The sedative properties of Benadryl may be lessened or negated when using with phentermine. In the same manner, Benadryl may lessen or negate the stimulating properties of phentermine. This is sometimes referred to as a pharmacodynamic interaction, where the response to one medication is affected by another. 

Benadryl (Diphenhydramine) Information

Benadryl (diphenhydramine) is a first generation antihistamine, which are noted for their relatively short duration of action (~4 to 8 hours) and their sedating properties.

In addition to its use for treating seasonal allergy symptoms such as a runny nose, Benadryl is also effective for treating symptoms of nausea, vomiting, and vertigo associated with motion sickness. As Benadryl has fairly significant sedating effects, it is often used as a night time sleep aide and is common component of over the counter sleep products such as ZzzQuil.

Benadryl begins to work about 20 to 30 minutes after taking by mouth and maximum blood concentrations are reached after 1 to 2 hours. The duration of action varies by individual but generally lasts 4 to 6 hours. Benadryl is generally well tolerated but can cause the following side effects:

  • Sedation
  • Dizziness
  • Respiratory depression (rare and generally only when combined with other medications that do the same)

Phentermine Information

Phentermine is classified as a sympathomimetic stimulant medication, chemically similar to amphetamines. It is most often used for the treatment of obesity and is one of the most commonly prescribed medications for weight loss. 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, phentermine should ideally be dosed in the morning to prevent issues of insomnia at nighttime. The most commonly reported side effects of phentermine include:

  • Headache
  • Insomnia
  • Overstimulation
  • Restlessness
  • Tremor

Taking Benadryl With Phentermine

As stated above, there is a minor interaction between Benadryl and phentermine. The effects ehac have on the body could be lessened or negated entirely if taken together. Benadryl could reduce the stimulant effects of phentermine while phentermine can lessen the sedative effects of Benadryl.

Ready for a more personal experience with your meds?