As far as I know it´s not the caffeine that causes the headaches. In fact you get a headache only if you QUIT taking it. But it´s said to be addictive and there´s some hint that this is really true: try not to drink your daily coffee tomorrow...
And for the amount of the acetaminophen you can take without trouble there´s really a big confusion. Some say the threshold for a rebound headache lies at 45g per month, which means 3 tablets a 500mg a day!!! And then there are others who think it´s not the total amount of drug that counts, but the number of days you take it. There should be twenty days off per month which leaves ten days with medication.
And then, the only way to find out, if you suffer from rebound headaches is to stop taking any OTC meds and wait and see if the headache reduces within TWO MONTHS. Bad thing to go through!
My experience is that caffeine tablets may help very well if I don´t take them too often. So I take them seperately and though I´m really depending on my daily coffee, I didn´t get addicted to them.:)
But if the amitriptyline doesn´t help try another preventive! There are several good ones and if one doesn´t work there´s still a good chance that another will.
Btw everybody sais it takes time untill the effect of a preventive sets on. But I always realize the first day I take a new medicine, if it reduces my headache or if it won´t even if I stay on it for month (I have a chronic daily headache). It may increase it´s effect over a week or so, but it´s there from the beginning...
The thing about rebound headache is, that everybody reacts different on the OTC medications so that it´s very difficult to draw the line. And then they don´t work the same in the same person at different times. Confusing as it is, I hope somebody will have better advice for you!