Husband 79 Years Old Began Doxycycline 100 Mg Twice A

Husband 79 years old began doxycycline 100 mg twice a day for sinitis, bronchitis and walking pneumonia, diagnosed Friday. He had dizziness and headache occasionally in the days before seeing doctor about symptoms, generally feeling lousy. Headache is pretty constant and bad last few hours. Dizziness comes and goes. While severe headache is considered grounds for immediately seeking medical attention, we aren't sure about whether this is nature of sin its or bad reaction to doxy. Husband feels should stay on meds to see if it can resolve infection. I am concerned about possible implication of headache. He takes warfarin. INR 2.2 the day he began the doxy. Any thoughts on the headache? Also had stage 4 kidney disease, not on dialysis. Thx