How can you show that the cream actually relieves the pain of diabetic peripheral neuropathy?

In order to see if the mannitol in QR cream really works to relieve the pain of diabetic peripheral neuropathy, we are comparing it to an identical cream without mannitol. This study uses two creams: one with only menthol, the other with menthol and mannitol. We believe that the cream with the added mannitol will provide more pain relief than the one without.

The people who are testing the creams will not know which cream they are getting and both creams will be tested on the same person.

If the cream containing mannitol provides significantly more pain relief than the cream with only menthol, then you know that it is the mannitol which provided the pain relief. 

Dr. Bertrand has already tested this cream on the burning pain caused by applying capsaicin (from hot peppers) to the upper lips. She showed that the mannitol cream was much more effective than the placebo in relieving this pain.[i]

[i] Bertrand H, Kyriazis M, Reeves KD, Lyftogt J, Rabago D. Topical Mannitol Reduces Capsaicin-Induced Pain: Results of a Pilot-Level, Double-Blind, Randomized Controlled Trial. PM&R. 2015;. PubMedID: 25978942