Hi Robin,
Lexapro is an SSRI and Effexor is a tricyclic.
As far as I know, tricyclics are better for neuropathy.
I don't see neuropathy mentioned here for Effexor
http://www.rxlist.com/cgi/generic/venlafax_wcp.htm
but feel free to have a look yourself in case I missed it.
So you have some choices, go back to the Lexapro and if it worsens, that
tells you something,
Change or increase the tricylic OR

I was just reading about neuropathy on a Lymphoma website
http://www.lymphomainfo.net/conditions/pn.html
"Some neuropathy may be permanent or increase after time - see (Late
Effects).. "
which takes me to this page
http://www.lymphomainfo.net/surviving/late-effects.html
which talks about a number of issues that might be a factor for your
brother.

You'd really have to tallk to his cancer doctor and ask for straight
answers. I would.
They know what treatments he's had and whether he has diabetes.
They would know if there's any chemo that might help (if that's what's going
on).
And if they don't know, if his pain is bad, he may have to be hospitalized
or as I said earlier, consult with Hospice, they're pain specialists.  They
can also offer practical "home tips" that perhaps can't be done in hospital.

However, they might also have to know (best guess) at what is or isn't
happening, so we seem to be going around in circles..
I really think this situaton is more this newsgroup can handle, especially
since there's so much unknown at this moment.
I do hope he obtains some pain relief soon.
J

If we are talking about neuropathic pain here when the antidepressants are
ineffective it is usual to try the anti-epileptic drugs. Sodium valproate is
usually good. Gabapentin is often very effective but expensive.
Transcutaneous Nerve Stimulating devices can also give good relief.
Neuropathic pain, however can be very resistant to all treatments and more
invasive things like local/regional nerve block or intrathecal analgesia may
have to be resorted to if the pain is severe or persistent enough to warrant
it.
MIKE