Cysteamine is a small sulfur-containing molecule that your body already makes when it breaks down the amino acid cysteine. It is naturally present in human skin and in milk.
On pigment it works several ways at once, which is probably why it performs better than its simple structure suggests. It blocks tyrosinase, the enzyme that builds melanin, and also peroxidase, another enzyme in the pigment pathway. It grabs the copper and iron that those enzymes need in order to work. It raises glutathione levels in the skin, and glutathione pushes pigment production toward the lighter, red-yellow kind of melanin rather than the dark brown kind. And it is a strong antioxidant, which matters because oxidative stress is one of the drivers of melasma.
The sulfur is not incidental. The thiol group - the sulfur-hydrogen pair - is doing much of the work, and it is also exactly what you can smell. There is no version of cysteamine that is both fully active and odorless. Formulators have reduced the smell considerably from the earliest versions, but they have not removed it, and honest brands do not claim to have.
Almost all of the trouble with cysteamine is irritation and smell. It is a well-tolerated treatment by melasma standards, which is a lower bar than it sounds.
Do not use it on broken, raw, sunburned or freshly resurfaced skin. Absorption and stinging both rise sharply, and irritated skin makes more pigment.
Do not use it around the eyes, on the lips, or on mucous membranes.
If you have very sensitive skin, rosacea or an inflamed eczema flare on the face, this is not the first thing to try. Start with azelaic acid, niacinamide or tranexamic acid and come back to cysteamine if those fall short.
If you cannot tolerate the smell, do not buy it. This is a real gate, not a squeamishness footnote. An expensive tube used twice is worse than a cheap treatment used daily for four months.
Pregnancy and breastfeeding: topical cysteamine has not been studied in pregnancy, and there is no safety data to lean on either way. Melasma is common in pregnancy and it often improves on its own afterward, so most clinicians advise waiting, using sunscreen in the meantime, and choosing azelaic acid if something active is wanted. That is caution in the absence of data, not a known harm.
Everyone else can try it. The decision is about tolerance, cost and smell, not safety.