Yes, it is safe.
Effexor does not interact with many other medications.
One of the medications,that should not be taken with Effexor,is Tagamet (Cimetidine), this is a medication ,used to treat ulcers.
In patients, with history of high blood pressure and liver disorders, Effexor can make worse these conditions.
And the other medications, that must not be taken with Effexor, are MAOIs (an antidepressive medications like: Parnate or Nardil) . This combination can be lethal. Because this combination,can produce seizures and tachycardia very intense and dangerous.
In relation,to the serotonin syndrome ,this is related, to MAOIs medications, not to Effexor or Prozac that are selective serotonin and norepinephrine reuptake inhibitors (SSNRIs).