What was the motivation for the prohibition on eating onions and garlic? I've heard two speculations:

1) Onions &amp; garlic are used to flavor meat, so they are guilty by association. I've heard the Jains have a similar prohibition for this reason.

2) Onions &amp; garlic incite the "passions"-- in other words, its a Indian herbal viagra.

Does anyone know the real motivation for prohibiting onions and garlic? Is the real motivation lost to the mist of time?

This is the 4th minor Precept in the Brahma Net Sutra.
Page 55 of this books tells a story about why no garlic

http://books.google.com/books?id=91OPcSUTFQ0C&amp;pg=PA55&amp;lpg=PA55&amp;dq=why+prohibition+of+onions+and+garlic+by+buddhist&amp;source=bl&amp;ots=-zabxvpxAT&amp;sig=GLbsLl7P1h6soNNWDG8SXrzfzM0&amp;hl=en&amp;sa=X&amp;ei=GN1OVISdDcz-yQS9q4DgCQ&amp;ved=0CFIQ6AEwBw

The text is from the Chanyuan Qinggui - the stories there are that the nuns were allowed to pick 5 heads of garlic a day but one day picked all of them. The Buddha then banned the use of garlic.

Another story tells that people who use garlic were not allowed near the Buddha when he was giving his sermon because of bad breath, body odor and possibly flatulence.

This seems more plausible to me if you have ever been around someone eating lots of garlic.

Onions, especially sweet onions, do not exhibit these same symptoms after eating.