So the answer to your question is that it should not be untreated to resolve on its own.
Some brain tumor resections are performed with patient awake and responsive to questions. This is to assure maximum resection of the tumor without effecting surrounding functional brain tissue. The patient may be asked to move extremities or report numbness if the surgeon is approaching normal brain tissue. This way he or she can remove as much of the tumor as possible.
Occasionally a patient may not tolerate this or may need to be put on general anesthesia in the middle of the procedure. In those situations the surgeon has be very cautious and may only be able to do a sub-total resection of the mass.
The answer to your question is that the duration of the surgery is very much multi-factorial and unpredictable.
Knowing what caused it, how ever, is just as important as the acute treatment.
The most common foods that can cause hives are nuts, chocolate, fish, tomatoes, eggs, fresh berries, and milk. Fresh foods can cause hives more often than cooked foods as cooking can alter some antigenic factors in food. Certain food additives and preservatives could also cause this reaction.
If this problem is recurrent you can see an allergist and do a blood or skin test for allergies.
Stress, substances, and mechanical pressure over skin can lower the threshold of histamine release. So given exposure to the cause the person would more likely experience hives. You can commonly see it under belt line, bra line, or areas of scratched skin due to the mechanical pressure over those areas.
In rare cases it could be a manifestation of systemic diseases but you must refer to your physician to investigate those.
Long term use of these drugs suppress sex hormones leading to loss of libido and other consequences of low sex hormones.
Opioids also lead to disturbance of serotonin causing mood disturbance such as depression, and sleep disorder.
Disturbance of other hormones such as cortisol and thyroid, cardiovascular stress, and osteoporosis are also among consequences of long term opioid use.
This may sound scary but if these medications are stopped on a timely fashion most of these problems are reversible.
Considering you have only been taking these for one year and the fact that you asked this question tells me you will most like have good prognosis and will work to find alternative therapies and avoid long term consequences.