Is there a risk to taking high doses of B vitamins?  I find they help
enormously for the fatigue I have suffered post-chemotherapy, but do
not wish to create new problems for myself.  The doses I am taking
are:  B1 110 mg; B2 10 mg; Niacinamide 130 mg; Niacin 10 mg; B6 10 mg;
B5 110 mg; B12 400 mcg; folic acid .4 mg; biotin 400 mcg.

This article may be of interest to you:

http://www.cancer.org/docroot/ETO/content/ETO_5_3x_Vitamin_B_Complex.asp

Yes, taking too much of a dose of vitamin b is a risk. You should
contact your doctor and ask how much is too much. Here is an article
to understand a little bit of vitamin b
http://news.bbc.co.uk/1/hi/health/4218186.stm

Vitamins including 
 B1(thiamine) ,B2(riboflavin), B3(niacin),B6(pyridoxine),
B12(cobalamin),, Folic acid & vitamin C( ascorbic acid)
Are water soluble & are not stored in the body , 
Vit B12 can be stored in liver in sufficient quantity .
 All of them rarely cause any toxicity because excess vitamins are
excreted in urine.