There is an LD (lethal dose) paper somewhere on PubMed, made quite some time ago already, where after they were finished testing lead and mercury individually they probably had a lot of rats (that's what they used, poor beasts) left over, so they tested the combination. Lethality jumped to a thousand times the lethality of either metal alone. Consider this general phenomenon, that combinations of substances in real bodies don't behave like they do when examined one by one e.g. to set "safe limits", whenever somebody claims the values are so low that it doesn't matter (not to mention of the impossibility of getting true body burden values in the first place without cutting off pieces from internal organs and sending them to a lab as the only reliable and also impossible method - and what you cannot measure does not exist, right?).
I've had a chronic mercury poisoning, fortunately, after self-diagnosing (but I was unsure, I just had no other options so I pursued the only one that made sense) I found a university clinic researcher doctor who thought the data I had from lab tests supported starting chelation to see if it helped. I got DMPS injected. Initially he said I'm in a "gray area" and that there must be something else. He never found anything else, and the chelation provided miracles. For example, I nodule I had had in my double-normal-size right thyroid for decades, unchanging, within a few months completely disappeared. Fortunately I had just gone to an endocrinologist a short time before to reconfirm that nothing had changed, and after there was a lot of "activity" in that area for weeks, starting after chelation, I went there again. The endocrinologist put me under the ultrasound twice because he did not believe it, but the thyroid was normal size and the nodule - gone.
There is a loooooong list of other things, from RSI, dry eyes, warts on the feet, winter depressions, cold hands and feet, psoriasis, digestion, sleep issues (incl. nightmares - my dreams now are incredibly beautiful and interesting, wow!), occasional cramps, longer and longer colds that in the last years before I was diagnosed only stopped in spring and during winter just didn't want to go away, etc etc. which all completely disappeared!
Every doctor ever, and myself too until I was forced to seek an answer because nobody knew what was going on and just declared me "fine", thought that each of those many many small issues was either normal part of live, due to aging, due to office work, due to "head" (imagination/psychology).
Things like that I had significantly decreased mental abilities and was more easily stressed I only recognized when they disappeared with chelation. Note that I called myself "healthy" until the very last day of escalation, when I was forced to start looking into it, and just like OP got not help from numerous doctors who all just went "by the book" - which has nothing about chronic heavy metal poisoning so it doesn't exist. (Also note that I don't fault any of those doctors, I would go again to every single one of the many I visited if I needed to, but now knowing better how to use their definitely existing and very high expertise.)
But I wanted to answer specifically about your concern about chelators.
After DMPS injections I switched to DMSA orally when I felt small doses over a long time where a far better fit for the remaining (and ever-decreasing) issues. DMSA is a key chelator for lead, secondary for mercury.
First, do look at the profile of the chelator in question. Some EDTA variants indeed remove vital things, but chelators like DMSA actually only remove the "bad" metals - and zinc.
However: I took tons and tons of DMSA for years, 100 mg every 3 hours ("Cutler protocol" - but I did experiments to verify that that recommendation indeed worked best for me, I didn't just take anybody's word any more and did a lot of self-experimentation, and my doctor, all those in my country, only have experience with DMPS injections ("pulse dose")). And yet, I never had any issue with zinc. I substituted a little bit, but far less than recommended. So even on an extreme chelation schedule I never saw any issues for the one "good" metal that both my chelators removed.
If you only do "pulse dose" it is a non-issue to begin with. Only a tiny fraction of whatever metal the chelator binds to is available to get into contact with it, most is stored somewhere. They all only work in extracellular space, and even there only reach a fraction of what's there. The stored metal, both good and bad, don't just slosh around in the watery solution of your extracellular body water or at the walls, most of it is inaccessible to a chelator. So only if you don something as extrem as I did with DMSA is it an issue at all.
Pulse dose DMPS injections, when there actually is a heavy metal to be chelated, indeed are a little hard on ones body, for a day or two. People in an already pretty on-the-edge condition may want to avoid it. I found combining DMPS injection followed by DMSA "a la Cutler" for a week helped. Just Cutler was/is far milder, and you can adjust your dose downwards (or start very low to begin with).
DMSA in food grade is pretty cheap when bought in bulk from China, who are the main makers of that chemical (Russians invented DMPS in the 1960s, but other countries make it too, in Germany e.g. "Dimaval" from Heyl, but that's not the main chelator for lead, so just FYI.
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By the way, even though I studied CS and business, I have a significant background in physiology, anatomy, chemistry/org.chem./bio.chem., statistics (and reading papers) so that I am very comfortable with all the basics, even if I know nothing about diseases (interest in how things work, but not in becoming a doctor who knows how things break). So I'm not prone to fall for wild esoterics but actually read papers and talked to scientists such as my researcher doctor.
What I find remarkable is that the problems of heavy metals are actually all known, for many decades. After all, the world abolished lead from fuel (except for small airplanes fuel) - quite a feat when they have trouble to come together to do anything else about subjectively more obvious issues. Also, you can find on many websites from organizations such as the NIH and other reputable sources, the medicinal threshold for lead is zero. Same for mercury. When I took a course about water treatment (Tufts University) the topic "lead" was a whole quarter of the course. A medical professor explained at length why zero is the desirable level.
So, people - doctors, officials, "the Internet" (commenters) - have no problem recognizing it as a problem. However, whenever there actually is a patient going to a doctor, heavy metals never play a role, unless it is one of the exceedingly rare acute poisonings, mostly industrial, and when you ask about it you are a nut case, and not all those ignoring well-established science.
"Cutler protocol" refers to recommendations from a man with a PhD in Chemistry named Andrew Hall Cutler aka Andy Cutler. He wrote some books about doing proper chelation and served as a chelation consultant for some years. I was fortunate to be internet acquainted with him and spoke with him once by phone.
His books are considered "gold standard." They are generally deemed to have better information than most doctors do about safe, proper chelation. You can tell they are still highly valued by people trying to cure themselves after all their doctors have failed them based on their high prices for used paperbacks:
https://www.amazon.com/Andrew-Hall-Cutler/e/B001K8VH30/ref=d...
I would like to emphasize that I did not actually read his books but mostly came to it through self-experimentation, comparing DMPS and DMSA and various ways to take them, and trying different ways more than once before forming an opinion (just for my own body and situation of course). I read the descriptions of the protocol on the Internet, which is simple enough: focus on keeping a somewhat even blood level of small doses of chelator by taking small (<100 mg max., much less at the start) doses every 3 hours during the day, every 4 hours during the night (lower metabolism allows to disturb ones sleep less). That's pretty much it.
I ignored all the rest, I think he recommends quite a few supplements. Also, I did not trust any person's advice. I tried it all on myself to see what actually worked. The only thing that I had clinical evidence for, according to my doctor, was the infrequent high-dose DMPS treatment, so for anything else I was on my own (not trusting Internet forum comments reporting success for this or that, including Cutler).
I did quite a few experiments, including a few times attempting to use DMSA capsules pulse dose, >500mg all at once and only one time, which a lot of doctors here (those few that use chelation) recommend. With that method I had significant side effects for days, unlike with the Cutler method. I found the minimum of three days that Cutler recommends to really be the absolute minimum, but I would set a better minimum at five days. I mostly did at least 7 days and up to three weeks at a time - with no little substitution of anything except for some (very little) zinc. No signs of a lack of zinc, ever, and just to be sure I read all I could about possibly symptoms. Of course, different people may have different results, apparently my body can compensate, probably by excreting less zinc and by getting more from food.
Also, I deviated from Cutler in two ways: One, he say that one should stop a round when one misses a dose by more than half an hour. I thought about it and the only way that that made sense was when I see it as a "psychological" rule, he wanted to prevent people from thinking "oh it doesn't matter" because a lot of people would drop a few doses if it was made too easy. However, when you genuinely miss a dose and ignore that larger problem it does not make sense to stop. Second, when my problems became less and my body "learned" the rhythm, and I didn't actually need an alarm any more, and when my body showed that it learned to "use" the rhythm, shortening the periods between waking up in times of greater stress all on its own (and more so at the beginning of sleep), I experimented with turning off the nightly alarm completely and letting my brain decide on its own whether it was better to wake up or to continue sleep past the four hour period. It turned out to feel much better. I have to emphasize that I only did this after my brain had proven for years that it had not just accepted the rhythm, but actively "used it", not just enduring it.
I've heard lots of bad stuff about high dose protocols and injections. It's been a lot of years since I was an active participant on a chelation list. I never pursued a formal chelation protocol, nor read any of Cutler's books.
But a word to the wise: Improper chelation has a nasty track record of making things worse. It's not really a good problem space for randomly trying stuff and seeing what works.
Here in Germany those doctors that offer it have significant experience with DMPS injections though. After my own experiences, I'd say that for many people - and I dare go beyond my experiences based on the experiences of those said doctors, which I know because my own doc, as a researcher, is in a good position to see them, and also is one of those offering courses for other doctors - DMPS injections seem to work pretty well at least for the hard initial part.
For example, there is a pattern that at first levels measured an hour after injection (DMPS is quickly excreted, mostly through kidneys) after the first couple of injections drop, but then suddenly jump up. That seems to be a sign that now the body starts its own "detox", which seems to have been overwhelmed and now, through the DMPS injections freed again. I had exactly this, after the fourth or fifth injection my values tripled after having decreased linearly, and my body became very "active" (including that thyroid, see my initial post).
I also had areas of jaw bone that were severely broken - a subcutaneous needle for buccal mucosa injection penetrated deep into the bone in several places, a very, very bad sign (you cannot penetrate bone with a small needle, not unless it's already severely disintegrating). My doctor injected DMPS right there, not deliberately breaking into bone, only where the needle easily penetrated on its own, with very little pressure. That started a lot of activity in my jaw bones, and after a year he could no longer penetrate the bone. My jaw bone issues, some of them existing for decades, continued to diminish and have now almost completely disappeared (and the rest will be gone too in time). All thanks to DMPS injections.
Opinion of those doctors here doing research and writing the books is that DMPS injections are orders of magnitude faster. From my experience I would agree. I still did (and do) DMPS injections, very infrequently, and there's a lot more happening all at once compared to low-dose DMSA. The method is heavily used here and experiences are good. Those doctors using it communicate quite a bit. I think if they saw danger they would have noticed by now. The method has been in use for decades now. Max Daunderer, a clinical toxicologist, came up with it originally during years of practice and that's the reason this is being used here so much.
Even more: During the harder years sometimes I ran into a road block with DMSA and Cutler. Then taking a DMPS injection quickly removed the symptoms and cleared the way to go back to DMSA and Cutler.