Folfirinox as you know consist of leucovorin+5FU+irinotecan+oxaliplatin; 5FU irinotecan and oxaliplatin all three can cause rash( esp. irinotecan =13%) and all three list hyperbilirubinemia(irinotecan=83%,oxaliplatin=13%,5FU=7%)
as side effect which can cause itching;5FU can also causes skin irritation;both 5FU and irinotecan have pruritus as side effect;apart from that neuropathy too.
Neurontin can be prescribed by your doctor for nerve pain if s/he agree ;
Benadryl cream and other OTC cream like hydrocortisone 1% cream for itching can also be used. Lainy as well as other caregivers on this board mentioned another cream(I forgot the name)which is very good for the rash too.
BTW, your regimen dose not include "targeted agents" like sorafenib or erlotinib or cetuximab,therefore there is NO relationship between "the worse the side effect of the rash is ,the more effective of the medication will be."
Please know that my personal opinion is not intended nor implied to be a substitute for professional medical advice. If provided, information are for educational purposes.Consult doctor is a MUST for changing of treatment plans.