Hi,
You have taking
tamoxifen for the last one year and three months, this can not be the reason for the itchy rashes on underarm, neck and anus.
The Doctor has prescribed you
Fluconazole means the most probable cause looks to be skin
fungal infection. The area which is usually obscured hence accumulation of sweat accelerated fungal infection.
I would advise you the following in such a situation:
Have
second opinion of a
Dermatologist that is skin specialist, investigations for diabetes or any immunity reducing conditions, medicines like you are taking that is Tamoxifen. Treatment of any primary cause will help to get you to get cured.
Proper personal hygiene like cleaning the area with plain water few times in a day to remove the sweat .
Use loose clothes and preferably pure cotton clothes.
Take care. Hope I have answered your question. Let me know if I can assist you further.
Regards,
Dr. T. Chandrakant, General Surgeon