Strontium ranelate, the prescription form of strontium that is not sold in the US, was restricted for awhile due to cardiovascular safety concerns. At that time some US supplement companies (like Doctor's Best) stopped making it. I believe it is now allowed in the UK (dangers disproven?) and is used for severe cases. https://en.wikipedia.org/wiki/Strontium_ranelate

The problem with strontium is that it distorts DEXA scan results. Strontium is heavier than calcium and when it replaces calcium in the bones, the DEXA reads it as denser.You may or may not care about that.

The algaecal dose is 680mg which is considered to be the therapeutic dose for osteoporosis.

After Tymlos, if I cannot tolerate Reclast, I may turn to strontium myself though I have some concerns. I was surprised that Keith McCormick did not recommend it though.