I am also surprised there hasn't been any trial on antihistamines as MS treatment. One can find questionable treatment with histamins (Prokarin or Procarin) but none for antihistamines against H1 type.
There was only one double-blind trial done with Prokarin. The results were published by a Journal at an University in Pennsylvania in 2001 (I think) and the conclusion was that Prokarin showed promise for treating the fatigue symptom of MS. (that was the primary end point of the trial) A secondary unplanned end point was the quick increase in the levels of A-Acetylaspartate which apparently is an important component in the brain that can have a positive effect on MS.
Unfortunately, our National MS Society in New York trashed the study before it was even published and even before they read the final draft. (They were given an early draft by a reporter who said he would not allow anyone else to see it prior to the official release by the journal involved.)
Any hope of more trials, which were recommended by the peer review, pretty much got destroyed since EDMS (company that produced Prokarin) did not have the resources for this and any revenue increase from a positive trial got stone walled. As well, the main component of Prokarin, histamine diphosphate) was way beyond any patent protection and there wasn't a drug company around that would touch it.
Interestingly, histamine treatment was done at a MS clinic in Washington State back in the late 40's and early '50s and supposedly about 80% of the patients reported positive results in their MS symptoms. Dr.Hinton Jonez, the doctor who was running this treatment, was at odds with the established MS community at the time and when he suddenly died in 1955 (?) all traces of his work disappeared and got buried. Too bad because who knows what kind of research may have been done on histamine.
And yes, Prokarin added the H2 component to a person's system.