what happens when prozac is taken with phentermine?

The use of fluoxetine (the active ingredient in Prozac) and
phentermine simultaneously is not recommended.

There haven't been many studies on the interactions of the two drugs,
but they do both affect appetite and thus could have a synergistic
effect. Here is the evaluation from DrugDigest.com:

Check Interactions
"FLUOXETINE may interact with PHENTERMINE
"Although the cause of this potential interaction is not clearly
understood, fluoxetine may increase the effects of phentermine. This
could cause an increase in side effects including irritability and
depression, restlessness, and anxiety. Other more serious side effects
include an increase in blood pressure, a rapid heart beat, and an
abnormal heart rhythm. If fluoxetine and phentermine are used
together, your doctor may want to monitor you closely for side effects
and adjust the dose of either drug if needed. Discuss this potential
interaction with your healthcare provider at your next appointment, or
sooner if you think you are having problems.
"This interaction is poorly documented and is considered major in
severity."
http://www.drugdigest.org/DD/Interaction/InteractionResults?drug=&cD=275&cD=539&dN=%22Fluoxetine%22+%22Phentermine%22+&CheckFDA=1

You can check other drug interactions here:

Check Interactions
http://www.drugdigest.org/DD/Interaction/ChooseDrugs

Here is the advice from Drugstore.com, although it is somewhat dated:

Ask Your Pharmacist
"Can I take phentermine and Prozac together to lose more weight? 
"'Phen-Pro' refers to a newer fad in weight loss medication
combinations. It combines the appetite suppressant phentermine (brand
name Ionamin or Adipex-P) and the antidepressant fluoxetine (Prozac).
Phentermine works in the brain to suppress appetite. A common side
effect of Prozac is reduced appetite. The small amount of information
available shows that Prozac alone may help promote weight loss, but
this needs to be confirmed by more research.
"There is no information about the safety and effectiveness of
combining the two drugs. So, the manufacturer of Prozac doesn't
recommend using these medicines together. Since the benefits and risks
of taking these medicines together aren't clear, your doctor should
monitor you closely if you take them together."
http://www.drugstore.com/qxa1234_333181_sespider-can_i_take_phentermine_and_prozac_together_to_lose_more_weight.htm

See also the statement at Drugs.com in the comment below by
Moghendhim-ga.

The official drug prescribing information for Prozac doesn't list any
interactions with phentermine. But it does say that patients should
not combine Prozac with other drugs except with a doctor's knowledge.

Prozac
http://pi.lilly.com/prozac.pdf

The official prescribing information for at least one brand of
phentermine advises against its use with selective serotonin reuptake
inhibitors, of which fluoxetine is one:

Ianomin Capsules
http://www.celltechgroup.com/Products/PI/Ionamin_R523.pdf

So the bottom line is that the safety of the two drugs taken together
hasn't been established, and there are indications that interactions
could include symptoms such as irritability, anxiety and heart
problems.

I hope you have found this information useful.

Sincerely,

Mvguy-ga





Google search terms used:

drug interactions
://www.google.com/search?q=drug+interactions

fluoxetine
://www.google.com/search?q=fluoxetine

phentermine
://www.google.com/search?q=phentermine

fluoxetine phentermine interactions
://www.google.com/search?q=fluoxetine+phentermine+interactions

Because some drug combinations can cause severe problems, I urge you
to phone a local pharmacy and query a pharmacist about this question. 
While doctors prescribe drugs, pharmacists are more fully
knowledgeable concerning drugs' effects and what drug combinations
should be avoided.

Best regards,
journalist-ga

Bad things, according to the drug interaction search engine on
www.drugs.com.

To quote:
[begin]
You have searched for drug interactions between the following drugs:
phentermine,Prozac.

Please note: only generic drug names are displayed.
fluoxetine and phentermine (major Drug-Drug)
Description:
GENERALLY AVOID: Several case reports suggest that patients treated
with serotonin reuptake inhibitors (SRIs) may exhibit an increased
sensitivity to sympathomimetic agents. The mechanism of interaction is
unclear. The reaction has been reported when fluoxetine was used
concomitantly with phentermine, amphetamine, or phenylpropanolamine.
Additionally, some sympathomimetic agents (e.g., amphetamines) may
possess serotonergic activity and should generally not be administered
with SRIs because of the additive risk of serotonin syndrome, which is
a rare but serious and potentially fatal condition thought to result
from hyperstimulation of brainstem 5-HT1A receptors. The interaction
occurred in a patient treated with dexamphetamine approximately 2
weeks after the addition of venlafaxine. The medications were
discontinued and the patient was given cyproheptadine for suspected
serotonin syndrome, whereupon symptoms promptly resolved. A second
episode occurred when dexamphetamine was subsequently resumed and
citalopram added. The patient improved following cessation of
citalopram on his own, and residual symptoms were successfully treated
with cyproheptadine. MANAGEMENT: In general, amphetamines should not
be combined with serotonin reuptake inhibitors. Close monitoring for
enhanced sympathomimetic effects is recommended if these agents must
be used together. Patients should also be monitored for signs and
symptoms of excessive serotonergic activity such as CNS irritability,
altered consciousness, confusion, myoclonus, ataxia, abdominal
cramping, hyperpyrexia, shivering, pupillary dilation, diaphoresis,
hypertension, and tachycardia.
[end]