What MSM is and is not
MSM stands for methylsulfonylmethane, an organosulphur compound. It occurs naturally in small amounts in foods and is produced synthetically for supplements; the molecule is identical to the natural one.
MSM is not a vitamin, not a mineral and not an essential nutrient with a defined requirement. There is therefore no reference value against which a sensible amount could be set.
The sulphur argument and its gap
The usual argument runs: sulphur is part of the amino acids methionine and cysteine, and these occur in connective tissue. The first part is chemically correct; the conclusion does not follow.
For an intake to do something, a deficiency would have to exist and be corrected. Sulphur deficiency is not described on a normal protein-containing diet. Consistently, no authorised EU health claim exists for MSM.
The research and its limits
Smaller studies on MSM exist, mostly with few participants, short duration and subjective endpoints. Reliable conclusions would require larger, longer and independently funded trials.
Small studies establish neither an effect nor its absence. The honest position is that the evidence is not sufficient for an authorised health claim.
Tolerability and comparison
At commercial amounts MSM is considered well tolerated; mild digestive discomfort or headache is occasionally reported at the start. Data for pregnancy, breastfeeding and children are missing.
Comparing products is simple here because there is only one form: the amount per daily serving is what counts. Purity statements and a per-batch certificate of analysis add further information.