vitamin P

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A water-soluble vitamin, found as a crystalline substance especially in citrus juices, that functions as a bioflavonoid in promoting capillary resistance to hemorrhaging.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of the water soluble bioflavonoids that promotes the resistance of capillaries to permeation.
• noun slang, euphemistic Prozac, an antidepressant.