Just came across this info for some of us who get very sick seemingly out of nowhere, when in fact we are having a toxic reaction to combined meds, when taking Cimetidine.  I know it happeded to me on many occasions.  No doctor ever put 2 and 2 together, but here's the proof.  

Drug interactions

Skeletal formula of famotidine. Unlike cimetidine, famotidine has no significant interactions with other drugs.
With regard to pharmacokinetics, cimetidine in particular interferes with some of the body's mechanisms of drug metabolism and elimination through the liver cytochrome P450 (CYP) pathway. To be specific, cimetidine is an inhibitor of the P450 enzymes CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4. By reducing the metabolism of drugs through these enzymes, cimetidine may increase their serum concentrations to toxic levels. Many drugs are affected, including warfarin, theophylline, phenytoin, lidocaine, quinidine, propranolol, labetalol, methadone, metoprolol, tricyclic antidepressants, some benzodiazepines, dihydropyridine calcium channel blockers, sulfonylureas, metronidazole,[8] and some recreational drugs such as ethanol (drinking alcohol) and methylenedioxymethamphetamine (MDMA).

The more recently developed H2-receptor antagonists are less likely to alter CYP metabolism. Ranitidine is not as potent a CYP inhibitor as cimetidine, although it still shares several of the latter's interactions (such as with warfarin, theophylline, phenytoin, metoprolol, and midazolam).[9] Famotidine has negligible effect on the CYP system, and appears to have no significant interactions.

I read Famitidine (Pepsid) is less toxic, but as I recall I tried them alland I can't take any of them.  I take a Xanax (Benzo) to sleep and calcium channel blocker for dysautonomia so they would interact with these.  I am not under control at all and feel like hell on a regular basis.  Like you Brigitte I don't know really what my diagnosis is other than Autoimmune Chronic Urticaria.  

Now I have complete malabsorption and no doctor to help me.

That's all I want too is a solid diagnosis.  I know I feel awful and I'm never going to be who I was before I got sick, but at least I can stop being completely consumed with "I DON'T FEEL GOOD".  That's all I've been saying for so long.

I have never heard of a mast cell disorder doc who recommended  cimetidine to MC patients.  Zantac (ranitidine) and Pepcid (famotidine) are the ones more safely taken as H2 blockers.

Some docs prescribe Claritin as an H1 blocker first, even though it is usually less effective, for the same reason.  It has fewer interactions with other meds.  Be sure to ask your doctor or pharmacist about any new medicine and whether it interacts with other drugs you take.

PJP, It's so frustrating not to have a diagnosis, as well as causing a lot of anxiety.  Unfortunately half the battle with these disorders is to find your own triggers (dietary and environmental) and avoid them.  The other is to get on the right combination of meds for your symptoms.

Once you have both of those things accomplished, you will still have flares of symptoms, but they will be shorter because you know how to medicate properly and milder because you will be keeping your "bucket" of triggers emptier by not exposing yourself to things that cause you symptoms.  The flares will also be less frequent.

If you don't have a mast cell specialist helping you or a doctor who is speaking with a mast cell specialist for advice, it's so important to find one.  It takes some of the burden off you when you have someone to help you get more stable.

Hope you find some relief.

Brigitte,  When you're having an episode, you might want to have your husband take your BP and check your heart rate.  There are inexpensive BP cuffs available.  Of course, take the Benadryl first.  If either is too high or low or irregular, call 911 or go to a hospital.  Also, you might want to take an H2 blocker with the Benadryl and some Singulair if you can tolerate it.