Pseudoephedrine
"I was told by my doctor to take Sudafed for allergy congestion. I guess she didn't know that it interacts BADLY with my synthroid and cytomel! (although she SHOULD have known - she is a doctor, after all.) After taking sudafed, I had a major anxiety attack and my blood pressure shot through the roof. It took HOURS for it to wear off. Apparently, there is a warning on the box that people with thyroid disease shouldn't take sudafed. Sudafed causes the effects of synthroid to increase dramatically and synthroid does the same thing to the Sudafed. If you take synthroid AVOID sudafed!! I threw the box away and will never take it again. I blame my Doc for not knowing this stuff is dangerous to thyroid patients."