66 = 66%?

No, it's not a percent.

It is just a scale. Out of 100 (with 100 meaning the most pain), the pain was rated at 66.

But it has said " set so that each individual rated the pain at 70 on a scale of 1 to 100" I wonder where the number 66 comes from.

More context:

http://www.sciencedaily.com/releases/2011/02/110226212356.htm

Plus, giving drug by needle injection through the "intravenous line"?

Can we see (from the expression of the author) that the drug is administered by injection (by needle)?

It comes from averaging. The patients all score differently so they reported the average pain level with and without the medication.

So, the author should use "about" before "70":

heat applied to the leg at a level where it begins to hurt -- set so that each individual rated the pain at about 70 on a scale of 1 to 100.

Am I on the right track?

Without reading the entire article, my take is that they raised the level until the patients rated the signal level at 70 in order to calibrate the energy level they wanted to use. If they run higher than that it might be unethical. Then they exposed all the patients to that level of energy. Of course 70 to some would be 60 to others, so the average reading was 66. Then they medicated a group and did the same thing, this time getting a lower value.

Ori - Patients are often asked to rate their pain from 1 to 10 (with ten being the most painful)

This experiment is using 1 - 100 to measure pain.

It started out with everyone saying that their pain level had reached 70 because heat had been applied to the leg. So pain was created to be at this level. This was a starting point, even though it was artifically created.

Then the medication was introduced and the pain level went down to 55.

They say that the patients' initial (beginning) pain level was 66. This was the real pain level of the patients. This was before they ratcheted it up to 70.