Bea, I use the drug interaction calculator at
http://www.drugdigest.org/DD/Interaction/ChooseDrugs to help
me with questions like yours.  Here's what they have to say
about herceptin and tamoxifen:

No interactions were found for the drugs you selected.
You searched for interactions between the following drugs:

Herceptin
Tamoxifen

I chose tamoxifen because it's pretty standard for ER+
cancers.  You may want to try with Arimidex.

If they're denying you treatment because of cost and you're
located in the U.S. I'd get in touch with whatever state
agency regulates health insurers in your state. - Tony

Bea wrote<< the doctor in Radiology department brought to my attention that
he found it odd that I was Her+ and ER+.  I asked if it could be an
error and he said no that it does occur in some patients.   >>

Hi  Bea, although it may be somewhat less common, I was also ER+ and Her2+  In
addition my main tumor was invasive lobular (pleomorphic) and supposedly only
5% of those with lobular are Her2+  (and only about 1o to 12% or 15% of breast
cancers ar lobuarI had lots of rare things going on.  
As far as getting Herceptin, what stage of breast cancer were you dx'd with.
At this time it has only been approved for  stage IV breast cancer unless you
are in a clinical trial.  If you want to know why your onc. did not consider
it, he should be the one that you are asking--directly. Are you  eligible for
any clinical trails?
It was recommended that I also take Tamoxifen.  However, Tamoxifen has a 25 to
30% failure rate and it may be for those who are also Her2+ as well as ER+ That
was why I refused Tamoxifen and the only way I was going to be able to get an
aromastase inhibitor through our health plan.
I did seek outside opinions and one top research oncologist told me that the
Tamoxifen might work effectively if Tamoxifen is also given.  However, the
aromatase inhibitors (i.e. Femara, Arimidex) may work better if given along
with Her2+

Responses interspersed.

There has to be a sensible return on expenditure.  Taking such a drug on the
off chance that it might help is just not sensible, especially as the
statistical benefit in this situation is not really known yet.  There are
also risks associated with this drug as with most others, and quite apart
form the cost, one has to do a risk balance on giving dangerous drugs to a
healthy woman for what is essentially prophylaxis.

Being HER+ is not really a "problem" per se.  Tamoxifen will help with the
cancer problem.  The tests indicate that your cancer cells have receptors to
both HER2/neu and Estrogen which both help it to bypass the normal controls
on growth.  Blocking either or both paths will reduce the stimulation to
grow and so increase your body's chances of getting rid of any stray cancer
cells before they have a chance to develop into a secondary tumour.

In the UK they would not usually do the HER2 test at this stage because the
result would not make any difference to the trreatment given.

The word "best" is dangerous, there are always diminishing returns to
consider.  Somewhere we have to come out of denial and draw a line saying
"this is how far it is worth going"

Tim Jackson