Very interesting decision from the one of the leading scientific publications to publish an article about Reiki therapy.
http://www.nature.com/news/consider-all-the-evidence-on-alternative-therapies-1.18547
[Edit: should be Nature publishes an article about alternative therapy]
Do you take the pills with water, and if so, have you tried just drinking the water? I find that water reduces a great many pains for me, including headaches, muscle cramps, and digestive difficulties, so it wouldn't surprise me in the least if the effects you're observing are from water taken with both the pain killer and placebo.
I do not take pills with water, but I have found that water, Emergen-C, and oral rehydration fluid do help in some cases, and I highly recommend them.
As far as I can tell, dehydration is a contributing cause to my headaches, but not the primary cause. Unsurprisingly, liquids help the most after I have been exercising.