Will taking tylenol once a day for 6 days before a blood test cause elevated liver enzymes? otherwise healthy, but dealing with a long term headache?

Acetaminophen (Tylenol and other brands) can be toxic to the liver, but normally only in very large overdoses. Most likely your liver function tests will be normal. But it is possible that some persons' livers might be more sensitive to it. If you have been tested with abnormal results, consider retesting when off the drug for a week or two. If uncertain, check with your doctor. Good luck.