A particularly paranoid person or a person going to a dangerous country may try this technique:

Purchase some type of herbs at the health food store that are cheap and come in size 0 capsules (well, if you just need something small like oxycontin, you can probably use 00 size.) Make sure its a really full bottle, if not, add more to it so that the capsules can't move around a whole lot. You could just stuff cotton in there too. Empty out the bottle. Take 20 or so capsules, carefully twist open while wearing latex or vinyl or whatnot gloves, dump out the herbs into a small folded piece of paper, insert as many pharmaceutical pills as can fit (you could probably get five oxy's in a 00 or one, maybe two watson 10/325's in a size 0), then fill in around them with the herb you dumped out. Close up and insure it is full enough that shaking it will not reveal the pharmaceuticals in the middle. Put these twenty pills in the bottom of the bottle and fill in the other 80 or more on top. There's also the potential to carefully remove the original foil wrapper and glue it back in place. Very difficult though. Anyway, I've never found this necessary, as I have never had any pill bottle with me opened flying in asia and back to the US several times. I had like 10 different bottles, many different prescriptions and herbs... never questioned on any of it. I guess the bottles could have been full of anything. But if you're paranoid, try the above method.