Hi Apes,
I tried nifedipine, then diltiazem then nitro.
Nifedipine and diltiazem are both calcium channel blockers. They work by preventing entry of calcium into the muscle, which blocks muscle contraction. Obviously they doesn't completely block muscle contraction, or your sphincter would be totally relaxed and you'd have no control. I think poor absorption plays a part in reducing the effect.
If your calcium channels are somewhat resistant to the drug, you won't get any benefit from it.
In my case, I seemed to be resistant to nifedipine, but susceptible to diltiazem, because dilt. did help me.
Nitroglycerine works differently. I forget the details, and I don't think its mode of action is fully understood, but it seems to be through the nitric oxide pathway. The idea is still to block muscle contraction, but by working on the myosin, not calcium channels. If you are resistant to nifedipine AND dilt, there is still a chance that nitro can help.
In my case, nitro did help, and gave me (I believe) many pain-free months. I still use it if I have a problem.
I believe (could be wrong) that the reason nitro gives people worse headaches is because it's extremely well absorbed. All three drugs are used to treat hypertension, by relaxing the smooth muscles in veins and arteries. All three can cause headaches by dilating blood vessels in the head.
Sorry if this is more than you wanted to know. I think the bottom line is that yes, it's pot luck what will work for you. If you haven't tried diltiazem, by all means try it. It does seem to cause allergic reactions more than the other two, but again, it's pot luck whether or not it will cause a reaction in you.
Have you tried heat? Because the mode of action is more or less the same - it relaxes the anal sphincter, enlarges the blood vessels and soothes the pain. There was a study done on using a heat pad to decrease the anal resting pressure. Here's the link:
http://www.ncbi.nlm.nih.gov/pubmed/10496555. I would bet that heat in conjunction with the drug will increase absorption of the drug, by dilating the blood vessels.
I'm sorry you are going through this, and having to make the painful decision. I'm in a similar situation and am going back onto nitro. I'm not in any pain at the moment but my fissure is still open, and the nurse wants me to take nitro for another six weeks. Boooo!