Parsley is a tasty herb available in curly-leaved and flat-leaf, or Italian, varieties. It can be an annual or perennial depending on the temperature of the region it's growing in, as it is not tolerant of very cold winters. Often used for garnishes and as part of salads, it has medicinal uses too. However, the food and tea forms, and supplements, have their side effects and cautions for use.
General Food Issues
Parsley eaten as a food garnish or chopped up in recipes generally isn't considered risky for healthy people. Obviously, if you find you have an allergy to parsley, it won't be good for you. Also, you must wash parsley thoroughly before using it -- and store it properly in the refrigerator instead of leaving it out on the counter -- to minimize the risk of food poisoning from bacteria residing on the leaves and stems.
Those who handle parsley a lot, such as cutters on farms, risk a skin reaction called photodermatitis. Drugs.com states that this is possibly due to compounds in parsley known as psoralens. If you are taking warfarin or lithium, speak with your doctor about whether or not you can eat parsley. The high vitamin K content within parsley may make warfarin less effective, and NYU's Langone Medical Center warns any use of parsley must be monitored by a doctor if you are taking lithium.
Uterotonic and Abortifacient Effects
Parsley contains two substances called myristicin and apiol, which may be able to affect the uterus and induce menstruation. The Langone Medical Center notes apiol has been investigated as a possible abortifacient. While these are more of a concern for parsley used as a supplement or tea, pregnant women should be careful not to eat large amounts of parsley. Note that no one has set forth a specific amount of parsley as safe for use in food. Your doctor should be able to give you further detail for your specific situation. Be wary of parsley essential oil, advises Drugs.com. While it has a reputation for regulating menstruation, taking parsley oil can result in kidney damage and neurological effects such as loss of balance, headache and seizures.
Resemblance to Poisonous Plants
Be very careful if you think you've found parsley in the wild, or if someone tries to make you eat parsley, drink parsley tea or take supplements they claim are from parsley they found. Parsley looks very similar to poison hemlock, which is fatal -- this is the same stuff used in the cup of hemlock that Socrates had to drink when sentenced to death in ancient Greece.