Do we know the mechanisms of action that cause the intense pain that myself and
@AllGoodThings have spoken about?
I've had water retention from other drugs but nothing has ever caused this level of discomfort and drop in QOL.
TLDR, if you have strong "sympathetic tone", your body releases excessive adrenaline from the ends of nerves in response to certain stimuli. Cold, emotional stress, *and drops in blood pressure*.
Amlodipine drops bp by blocking the calcium "L" channels in medium and small blood vessels, but not tiny and microscopic vessels.
In response to this drop in bp, everyone, but particularly those with strong "sympathetic tone" release adrenaline from the ends of nerves, which among other things cause those tiny vessels to contract in an attempt to raise blood pressure and reduce blood flow to prevent blood from excessively cooling in extremities, to conserve energy.
The "fight or flight response".
The problem is you now have high blood flow going into tiny constricted vessels, stretching them out until water leaks out of their walls. This local tissue water retention (aka edema) can put pressure on nerves, and you feel pain.
The degree to which this happens depends on individual biology; and reactions like yours are rare but not unknown.
Cilnidipine blocks N calcium channels (in addition to L like amlodipine), preventing the release of adrenaline (noradrenaline aka epinephrine to be specific) that occurs during the sympathetic response (again, to stress, cold, or blood pressure drops), so small vessels stay relaxed , wide open and don't stretch, which prevents water from leaking out.
Blocking N channels also prevents increases in heart rate, and tubules in the kidneys from contracting that also occur when the sympathetic "fight or flight" response kicks in.
The cool thing about this is that it's very targeted, only blunting excessive sympathetic response, without the sedation other methods cause (like beta blockers, which block adrenaline receptors, rather than blocking adrenaline release via N channels).
If you're a naturally "high strung" person, you'll notice your body doesn't respond the way you're used to when you get angry, ie muscles tensing, bp rising, but otherwise, you don't feel any different.