In contrast to the comparatively well-developed Canadian discount drug market -- an area which has been positively flooded with carbon copy 'me too' sites attempting to cash in on the rush to buy discount drugs abroad -- the market is comparatively under-developed. Like Canada, Mexico has long attracted regular forays from citizens living along the border who journey a few miles to make big savings on prescription drugs. But the online trade in discount drugs is only just getting started in earnest; I would expect the market to be awash very soon with the same kind of low quality profit engines that have sprung up all over Canada. (I would also expect further proliferation of the widespread scams offering to sell you special 'members only' access to lists of pharmacies.)
Having said that, I am aware of at least a few reputable online discount pharmaceutical ordering services working with suppliers in Mexico. They offer what may be the lowest prices available anywhere.
Note that drug laws in Mexico permit many drugs to be dispensed without a prior prescription; this enables pharmacies to ship (legally, in accordance with law) many medications which US pharmacies would not be able to ship without a prescription.
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