Don’t harvest herbs near farmers’ fields or in irrigation ditches.
Have a look at this lesson for tips on how and where to harvest to avoid toxicity.
Harvesting, Drying & Processing Herbs
If one is prone to kidney stones oxalates can be an issue. Lots and lots of good plants contain them. As long as they aren’t consumed in high doses or long term, its not an issue.
Don't use herbs or combine herbs with medications or use them during lactation or pregnancy without talking with your healthcare provider.