I Took One Dose Of A Nitrate Drug About 3

I took one dose of a nitrate drug about 3 pm because i had shortness of breath after climbing a flight of stairs. Later in the evening I started to get a headache, which I was told might happen. The next day I took half of a pill. The headache got much worse. Today I did not take any of the medication and the headache continues. How long does it take to get the medication out of my system? Any suggestions?