A rash showed up mid-chemo cycle. What now?

Call 911 or go to an emergency department if a rash appears suddenly alongside swelling of the face, lips or tongue, or any trouble breathing. NCI notes that a sudden or severe rash, hives, or a burning sensation can be signs of an allergic response to chemotherapy. Breathing trouble or swelling turns that into an emergency.

Call your care team the same day if the rash is severe, spreading over a large part of your body, blistering, or painful, and especially if it involves your eyes or the moist linings of your mouth while you are on immunotherapy. NCI names those exact features as reasons to call your doctor for advice.

Also call the same day if the skin in a radiation area becomes wet, sore or looks infected, which NCI describes as a moist reaction.

Most rashes during chemotherapy are not emergencies, and it helps to know that before you look at your arm at 11pm.

NCI describes the usual picture: some types of chemotherapy can cause skin to become dry, itchy, red or darker, or to peel. Minor rashes are on the list. So is sensitivity to sun and changes in skin colour. Nails may darken and crack.

Targeted therapy drugs have their own pattern of dry skin, rashes and nail problems. If you are on radiation, skin in the treated area can become dry, peeling, itchy, red or darkened, looking rather like sunburn, and it can swell.

A minor rash is common enough that it is worth reporting but not worth panicking over.

Do not stop your treatment because of a rash without talking to your team.

NCI is explicit about this for targeted therapy: medical consultation before discontinuing therapy is important if a rash develops. A rash is information, not automatically a reason to abandon a drug that is working. Your oncologist may have options that keep you on treatment.

NCI's self-care list is short and consistent.

• Use only gentle soaps and skin products that your team recommends.
• Apply recommended creams and lotions to head off dryness.
• Take lukewarm showers rather than hot ones, and put moisturizer on while your skin is still slightly damp.
• Avoid alcohol-based or fragrant products.
• Keep nails short and clean, and wear gloves for household tasks.
• Skip manicures and pedicures while you are in treatment.

The lukewarm-and-still-damp detail is easy to dismiss and does more than most people expect. Hot water strips oils. Moisturizer on damp skin traps the water that is already there.

Because some chemotherapy raises sunburn sensitivity, NCI recommends sunscreen and sun-protective lip balm, plus long sleeves, long trousers and a wide-brimmed hat outdoors.

This applies on overcast days and on short errands. A burn on skin that is already fragile is a much bigger setback than a hat is an inconvenience.

The radiation-specific instructions are different, and they matter.

NCI says not to use heating pads, ice packs or bandages on the treatment area. Powder and antiperspirant are to be avoided, as is frequent shaving of tender skin. Keep the treated area clean and dry to lower the chance of infection.

Skin folds are where moist reactions tend to develop, so those areas deserve a daily look.

Describe three things: where it is, what it looks like, and how it feels.

Where it started and whether it is spreading tells your team about the pattern. Whether it is flat, raised, blistered or peeling tells them about severity. Whether it itches, burns or hurts tells them how much it is costing you.

Photographs on your phone, taken in the same light each day, are genuinely useful. A rash that looks alarming on Tuesday and better on Thursday is a different conversation from one that has doubled.