I've been told mangoes can cause a rash like poison ivy. Is that true?

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The mango is in the same plant family as poison ivy and its peel can indeed cause allergic skin rashes similar to poison ivy. That's especially true for people who are particularly sensitive to urushiol, the chemical involved. For this reason, it's best to peel a mango before eating it!

Foods plants, by definition, are supposed to be safe to eat, but one has to be careful. Potatoes are fine - unless they are green; those have a toxic substance called solanine, which can be fatal in large doses. Nutmeg in large quantities has unpleasant hallucinogenic effects followed by a very severe hangover and liver toxicity.

The cashew nut, before it's cleaned and roasted, is wrapped in a very nasty corrosive part of the cashew fruit that must be avoided. Fiddlehead greens have been implicated in stomach cancer, but are probably OK in reasonable quantities. Nettles, though not eaten often in the United States, are delicious when cooked, but deliver nasty stings when fresh.

Parsley is also toxic in large quantities, and should not be eaten in excess by pregnant or nursing women. I've seen it on an Italian pharmacy's list of toxic plants!

And the list goes on . . . but keep in mind that dose is everything. Salt and even water can kill you if consumed in excess.

This Dr. Knowledge entry was written by physicist John Swain of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.