Our son had massive difficulties with medications. After observing his reactions we chose a pattern of always starting any new medication at below recommended dosage to test tolerance first prior to ramping up to "therapeutic" level. Many medications we were unable to even get to the recommended level. If they did not work, we stopped them. Some medications need to be given time and as long as they did not make him worse we stuck with them long enough to see. In the end he only really tolerated and appeared to benefit from 3 medications: Zoloft at very minimal dose, Celebrex at lowest dose possible and Prilosec. All other medicines either created negative responses (Topamax, Immetrex, Depakote, Cymbalta) or were not effective long term (Indomethacine, Provigil, Wellbutrin,).
To me the negative responses to medications appeared to be two fold. One was side effects to the medication such as the hives he developed while taking Cymbalta. The other response was hypersensitivity (which I think also occured with most side effects). This hypersensitivity was really across the board with all medications, but some were more tolerable or gave us results so it was worth pursuing. My opinion on the hypersensitivity is that his Nervous System was highly sensitized (True Central Sensitization not the UK NHS BS) and created a whole body sensitivity including the processes and organs the medicine was intended to help.
Interestingly now that he has made an apparent full recovery, he has very "normal" responses to medications and is able to tolerate normal doses of medications (at least to the ones he has taken recently). I also think that the hypersensitivity will be something researchers such as those at Stanford and in Norway will have to take into account especially treating those who have severe ME/CFS. They may have to slowly introduce new medications and ramp up over time to not overwhelm the patient's system. That would mean using "Sub-therapeutic doses" and gradually increasing over a period of months. For those with Mild ME/CFS it seems to me that they could go a little faster in upping the dose, but if done too aggressively could create the crashes many have mentioned.
Just my thoughts on observations and what I know about how the body functions as a whole. The particulars will have to be determined by the researchers and physicians.