THE ANSWER
Keep the area clean, leave it alone, and skip makeup, heat, heavy sweating and sun on the treated area for the rest of the day. Expect redness for a few hours up to a day or two. If anything looks outside that range, or gets worse rather than better, see a doctor.
Why
Treatment leaves each treated follicle temporarily open and the surrounding skin briefly irritated. Aftercare is about not introducing anything into that window and not adding further irritation while it settles.
Redness is the normal response and it fades. Small raised spots can appear at treated follicles and settle over hours. Neither is a problem in itself.
Heat, sweating and friction all increase irritation in an already irritated area, which is why the usual advice covers exercise, hot showers and tight clothing on the treated area for the rest of the day. Makeup and heavy products are avoided for the same reason.
Sun matters because irritated skin is more prone to pigment change, particularly in skin that marks easily. This is more important, not less, for people with darker skin tones.
Picking at the area is the one thing that genuinely raises the risk of a lasting mark. Small spots settle if left alone.
What should I not do after a session?
Avoid makeup on the area, hot showers and baths, saunas, swimming, heavy exercise, tight clothing on the treated area, and sun exposure for the rest of the day. Do not pick at the area.
How long does redness last?
A few hours for most people, up to a day or two on more sensitive skin or after a longer session. It settles on its own.
Can I wear makeup after?
Better not to on the treated area for the rest of the day. Applying makeup means pressing product into skin with open follicles.
Can I use deodorant after?
Skip it on the treated area for the rest of the day after underarm work, for the same reason.
Can electrolysis cause an infection?
It is uncommon, and keeping the area clean and unpicked is what keeps it uncommon. If an area becomes increasingly painful, swollen or hot rather than settling, see a doctor.
What the evidence says
These are standard aftercare instructions given consistently across the field. Individual practitioners vary in specifics, and your own practitioner's instructions take precedence over general guidance. This page does not recommend any product.
What to do next
Plan sessions for a day when you do not need to look your best that evening, and when you are not exercising afterwards. That solves most aftercare problems before they happen.
If your skin marks easily, aftercare matters more and it is worth discussing at consultation. See hair removal for darker skin tones and does electrolysis scar.