I had a reaction once and you'll know it when it happens. I was lax about the diet and one time at a restaurant had pasta with vodka sauce which contained real, aged cheese (processed would not have been so bad).

All of a sudden I had a migraine level headache that started in the back of my head and moved toward the front. Its called a traveling headache and it's a really strange sensation. I felt it literally moving until It stopped at the top of my head. apparently it can continue to the front (in the forehead) which is more dangerous, so I was lucky.

Also, I was hot, very nauseous, my pupils were dilating and my neck was suddenly very stiff. I could not turn it at all from side to side. I should have gone to the ER but figured I'd end up waiting forever so didn't bother. For about a week after I had a mild head/neck ache whenever I took the Parnate so I had to stop taking until all my headache was gone. My doctor made me promise to go to the ER if it happened again- they have a pill that counteracts the rise in blood pressure and works very quickly. Anyway, after it wore off I was able to go back to my usual dosage and never had another issue. It was scary at the time though.