Selenium sulfide

Almost every entry in the anti-dandruff monograph is a range. Coal tar runs 0.5 to 5 percent, pyrithione zinc 0.3 to 2 percent, sulfur 2 to 5 percent.

Selenium sulfide is not a range. The regulation permits “selenium sulfide, 1 percent” and “selenium sulfide, micronized, 0.6 percent”. Two exact figures and nothing between or beyond them.

Why the fixed numbers

A range exists where a formulator has room to trade strength against tolerability. Two fixed points mean the evidence supported those specific configurations and nothing else was established.

The micronised entry is the more interesting of the two. Grinding the particles finer increases the surface area available to act, so a lower concentration does comparable work. That is a formulation improvement written into a regulation, which is rare enough to be worth noticing: 0.6 percent micronised is not a weaker product, it is the same job done with less material.

Practically, comparing a 1 percent product with a 0.6 percent micronised one on the number alone will mislead you.

What it is for

Seborrheic dermatitis and dandruff, both driven substantially by Malassezia, a yeast that lives on everyone’s skin and causes trouble in some people. Selenium sulfide acts against it and slows the turnover of scalp skin cells.

It is generally considered among the more effective options in the group, which is why it persists despite two real drawbacks.

The two drawbacks worth knowing

Hair colour and texture. Selenium sulfide can affect coloured, bleached or chemically treated hair, and it can leave a residue on light or grey hair. This is the member of the anti-dandruff group most likely to change how your hair looks, and it is the reason people abandon it.

Dryness. The dermatology guidance for high curl pattern hair groups it with Pyrithione zinc and Coal tar as products to use carefully “to avoid hair shaft damage and dryness”.

The same guidance gives the fix, and it is the same one that applies to the whole group: apply it “directly to the scalp rather than the hair shafts”. The active is working on the scalp. Running it down the length delivers it where it is not needed and dries the fibre out.

Where it sits against the alternatives

Pyrithione zinc is the everyday default and the easiest to live with. Selenium sulfide is a step up in effect and in inconvenience. Coal tar is the option kept for cases the first two do not settle, and it is the most drying of all of them.

None of that is a ladder to climb on your own. Rotating between actives is a common clinical suggestion because the organism responds differently to each, and that is a conversation worth having rather than a shelf to work through.

The full monograph table, with every active and its permitted concentration, is in the list of ingredients that are legally drugs.