First of all here are some links about Decadron, including the positive and negative information about Decadron.
http://www.drugs.com/pro/decadron.html
http://www.ncbi.nlm.nih.gov/pubmed/688248
http://organizedwisdom.com/Dexamethasone
We need to remember that every drug has a positive side and a negative side since every drug, including those meant to prevent various side effects can have side effects of its own. We just have to weigh the pros and cons of everything we take.
Every time a medical professional wants to prescribe Decadron to me I refuse. Their usual excuse is to prevent nausea, so I tell them to prescribe a big bottle of anti nausea pills and forget the Decadron. I always get the pills, but I have never used any of them. It is nice to have "insurance" at home in case I need it. Ginger also works for nausea. Smile.
I also refuse Benedryl.
There are also alternatives for fluid build up and whatever else Benedryl and Decadron are supposed to do. All you have to do is ask your doctors what they are or do a search on the internet.
Good luck with the Gemzar. I really, really hope it works for you. Take care.