Selenium sulfide does two things at once. It is antifungal — it kills and slows the growth of Malassezia, a yeast that lives on everyone's skin and feeds on skin oil. Malassezia is not an infection you catch. The problem is that some people's immune systems react to the yeast and its byproducts, and that reaction is what produces the flaking, itching and bumps. Fewer yeast means less for your skin to react to.
The second effect is what separates it from zinc pyrithione. Selenium sulfide slows the turnover of skin cells and reduces how much oil the skin makes, so an oily scalp gets less oily. Since oil is what the yeast feeds on, cutting the supply removes the fuel as well as the yeast. That is why it usually outperforms the gentler washes on a scalp that is greasy as well as flaky — and also why it can leave dry or sensitive skin feeling stripped.
Most problems with selenium sulfide are irritation, smell and staining rather than anything dangerous. It has been in use for decades.
Do not use it on skin that is broken, weeping, raw or freshly burned. Absorption rises sharply through damaged skin, and it stings badly.
Do not use it on children under two without a doctor's direction.
If your hair is blonde, grey, white or chemically coloured and you cannot risk a tint change, choose something else. Zinc pyrithione and ketoconazole do not discolour hair.
Avoid it if you have known selenium sensitivity or have reacted to a selenium sulfide product before.
Pregnancy and breastfeeding: topical use on the scalp is generally considered acceptable, but the safety data are thin, and most clinicians will steer you towards ketoconazole shampoo or zinc pyrithione instead. That is a caution based on limited evidence, not on a known harm — worth saying plainly rather than dressing up as a warning.
Everyone else can use it. The real decision here is about tolerance and hair colour, not safety.
Week one. Itch usually settles before the flaking does. If the itch is easing, it is working even if you can still see flakes. Oily scalps often feel less greasy within a few washes.
Two to four weeks. Flaking drops off noticeably. This is the point most people would have given up, which is why so many people believe medicated shampoo does not work on them.
Tinea versicolor, two to four weeks. The scaling and itching go quickly. The colour does not. The pale or dark patches left behind are pigment change, and they take two to six months to even out — often longer on brown and Black skin, where the contrast is more visible. Flat patches with no scale are not treatment failure and do not mean you need more shampoo.
Fungal acne, three to four weeks. The itchy, uniform bumps on the chest, back and hairline flatten out. New crops stopping is the first sign it is working.
Four to six weeks with no change at all. That is the point to stop and reconsider the diagnosis rather than buy a stronger bottle.
When you stop. It comes back. Malassezia is a permanent resident of your skin, not an infection you clear. Tinea versicolor recurs in the majority of people within a year without maintenance washing, and dandruff returns within weeks to months. Maintenance is a wash once a week or once a fortnight — a small chore, and the whole difference between controlled and recurring.