What does the M in μM-Sux stand for?

I am looking at some data regarding a study around a channel-blocking agent called suxamethonium. 


The histograms of the open times of the channels at varying concentrations of suxamethonium are given. However, I am having a difficult time figuring out the dosage. Here are examples of dose listing formats:


* 200 μM-Sux
* 500 μM-Sux


etc.


I understand that μ in this context is to mean micro and I believe the suffix "Sux" is supposed to stand for suxamethonium. But what is the capital "M"? I looked it up in a scientific notation table and it stands for mega. Am I supposed to infer that this is a liquid (i.e. ML)? 




You are right assuming that μ is a [metric prefix](https://en.wikipedia.org/wiki/Metric_prefix), and that the number denotes the concentration of suxamethonium (suxamethonium chloride, I presume). However, this is supposed to be small caps "M" (as $\rm \small M$), not capital "M" (as $\rm M$). Also note that use of the term "[molarity](https://en.wikipedia.org/wiki/Molar_concentration)", as well as its notation $\rm \small M$, is discouraged [[1, p.27]](https://www.nist.gov/physical-measurement-laboratory/special-publication-811): 



> 
> The term molarity and the symbol $\rm \small M$ should no longer be used because they, too, are obsolete. One should use instead amount-of-substance concentration of $\ce{B}$ and such units as $\rm mol/dm^3$, $\rm kmol/m^3$, 
>  or $\rm mol/L$.
> 
> 
> 


And, of course, $1\,\rm {\small M} = 1\, mol/L$.


Also, [check out the answer](https://chemistry.stackexchange.com/a/49633/41328) to a similar question "[What does the unit M stand for?](https://chemistry.stackexchange.com/questions/49632/what-does-the-unit-m-stand-for/49633)"


### Reference


1. [Thompson, A.; Taylor, B. N. Guide for the use of the international system of units (SI). *NIST Special Publication 811* **2008**.](https://www.nist.gov/physical-measurement-laboratory/special-publication-811)


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The capital M is simply an abbreviation or another way to denote mol/L, or moles per liter.


More specifically, the 'M' stands for the word 'molarity', which as a physical quantity is defined as:
$$\ \frac{\text{Amount of Substance of Solute}}{\text{Volume of Solution}}$$


Therefore, $\pu{200 μM}$-Sux means $200$ micromoles of suxamethonium per liter, or $\pu{0.000200 mol/L}$ [because the micro (μ) symbol denotes $10^{-6}$].


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