Cite? Without a prescription (and why would you have one, since they're OTC in Canada?), you're limited to 50 tablets. From the horse's mouth:

A new bill was recently passed by Congress that amends a portion of the Controlled Substances Act (21USC956(a)). This amendment allows a United States resident to import up to 50 dosage units of a controlled medication without a valid prescription at an international land border. These medications must be declared upon arrival, be for your own personal use and in their original container.

(source: Customs' website)

Hence a previous poster's statement about Wal-Mart selling no more than 50 tablets to U.S. residents.

I was severly hassled by U.S. Customs and had my car searched for bringing back one bottle containing 100 tablets. I offered to dump out 50, but they let me keep all 100 with a warning to keep it under 50 in the future. (I got the impression that they really don't want people bringing back the stuff in any amount, legal or not.)