According to the MIMS (Monthly Index of Medical Specialities- a pharmaceutical prescribing reference guide) there is no known interaction between Phentermine and Tramadol hydrochloride.
However some sources (less reputable than the MIMS) suggest decreased seizure threshold which I imagine would be more likely if you have a history of seizures, take other medications, illegal substances, alcohol, very old etc.
Having said that, I know Tramadol also has SNRI/SSRI properties (it’s not strictly classed as an SSRI/SNRI hence it may not come up in a drug interaction search- hence why I’ve researched these separately later on) and anxiety can be a side effect. Given Duromine can have the same effect despite working on different neurotransmitters (weakly on serotonin) this effect could be compounded when taken together- theoretically.
Generally there exists interactions between SSRIs and Phentermine (as with amphetamines and SSRIs).
There is a moderate interaction between amphetamines (the illegal kind, that’s chemically similar to Phentermine but usually at much higher doses) and SSRI that may cause increased serotonin and therefore increased risk for seretonin toxicity/Serotonin Syndrome (very rare side effect in general) as well as high blood pressure (and things that occur as a result of that like cardiovascular valve damage).
However this survey for example failed to identify any such incidences in 1174 patients (which I can’t link due to a forum rule). Make of that what you will. But the big picture is that it’s extremely unlikely and most evidence is anecdotal.
That’s all I’d be aware of- the SNRI/SSRI aspect on cardiovascular health as well as mood and behaviour, anxiety, dry mouth etc. But that isn’t thoroughly proven- just theoretical. The opioid-like properties of Tramadol strictly speaking do not have a known interaction with Phentermine to speak of. Of course, it may make you drowsy/less alert than Phentermine would otherwise.
But I’m sure you’ll be talking to your doctor and pharmacist anyway!
(Please don’t ask me why I’m researching this so thoroughly, I’m bored)