I have discovered that Alleve (I take it for knee and hip pain) has a big impact on MY Heart Rate while riding. After not taking it for awhile, I took two tablets before a charity 22 miler this morning. My heart rate was at least 10 beats higher than it has been recently for a similar effort.
It was so much higher that I really did not know how much effort I was exerting - so I just rode by feel and did OK. Averaged about 17 mph.