The cream contains aminolevulinic acid, a substance your body already uses to build a molecule inside cells. Abnormal and rapidly dividing cells take it up faster than healthy ones and convert it into protoporphyrin IX, a chemical that reacts strongly to light. After one to three hours, those damaged cells are loaded with it and the surrounding normal skin is not.
Then the light goes on. Blue or red light at the right wavelength hits the protoporphyrin IX, which releases a burst of reactive oxygen inside the cell and destroys it from within. Because the selectivity comes from which cells took up the cream, the treatment finds abnormal cells across the whole treated field — including early patches that have not become visible yet. That is the real argument for it over freezing individual spots: sun-damaged skin does not have a neat edge, and this treats the whole area of damage rather than the bits you can point at.
Red light reaches deeper than blue and is used for thicker lesions and superficial basal cell carcinomas. Blue light is common for actinic keratoses on the face and scalp. Depth is also the limitation — the light and the cream only reach so far, which is why an invasive tumour cannot be treated this way.
Book the appointment so that you can be indoors, away from windows, for about 48 hours afterwards. This is the part people underestimate and the reason most bad outcomes happen.
Arrange a ride if the treated area is your face — driving home in daylight means sitting in a car full of light.
Have a wide-brimmed hat, sunglasses and long sleeves ready for the trip home, and cover the treated area with clothing rather than sunscreen.
Stop retinoids and exfoliating acids about a week before, unless your dermatologist tells you otherwise.
Avoid sunburn in the treated area beforehand.
Tell your dermatologist about any medicine that makes you sensitive to light, and about any history of cold sores if your face is being treated.
Come with clean skin and no makeup, moisturiser or sunscreen on the area.
Expect to be at the clinic for two to four hours in total. Most of that is the wait while the cream absorbs. Bring something to do.
Ask what pain relief they offer during the light. Cold air, a fan, pauses, and sometimes a nerve block are all reasonable and normal to request.
The area is cleaned, and often lightly scraped or rubbed with a rough pad or acetone to help the cream get in. Thick, crusted patches may be gently scraped down first. This part is mildly uncomfortable, not painful.
The cream is painted on and the area is covered. Then you wait — one to three hours, depending on the drug and the area. Most people wait in the clinic. You will feel nothing during this time.
The cream is wiped off and you are positioned in front of the light with your eyes protected.
The light runs for about 8 to 20 minutes. This is the part to prepare for. Most people describe a sharp stinging or burning that builds over the first few minutes and gets intense — it is commonly rated more painful than expected, and it is one of the more uncomfortable routine dermatology treatments. A fan blowing cold air on the skin helps a great deal. Ask them to pause if you need it; a pause does not ruin the treatment.
When the light goes off, the pain drops away quickly. Within minutes it is usually down to a hot sunburn feeling.
The area is covered and you go home with your skin protected from light.
Hours 0–48: the critical window. Your skin is still loaded with the light-sensitising drug, and any bright light — sun through a window, a car windscreen, a bright kitchen, some office lighting — can trigger a severe burn. Stay indoors, keep curtains closed, keep the area covered with clothing. Sunscreen does not protect you adequately here. Physical cover does.
Days 1–3: the treated area is red, swollen and hot, like a strong sunburn. Facial swelling around the eyes is common on day two and looks worse than it is. Cool compresses and plain petrolatum help. Paracetamol or ibuprofen is reasonable.
Days 3–7: crusting and peeling. The treated patches darken, scab and flake off. Do not pick. The skin underneath is fragile.
Days 7–14: most people look normal again, sometimes with lingering pink patches. Some redness can last three to four weeks after a strong treatment.
A useful thing to expect: patches you never noticed will light up, redden and crust. That is the treatment finding damage that had not surfaced yet. It means the area was more sun-damaged than it looked, not that something went wrong.
Sunscreen daily on the treated area from about day three onwards, for good.
Once the crusting settles, usually two to four weeks after treatment, the area looks visibly better: fewer rough patches, more even colour, and often smoother texture as a side benefit.
For actinic keratoses, a single session typically clears about 70 to 90 percent of the patches in the treated field. A second session a few weeks later is common and pushes that higher.
For superficial basal cell carcinoma and Bowen's disease, clearance rates are good but lower than surgery. Recurrence over five years is meaningfully more common than with excision or Mohs surgery. That is the trade being made for a better cosmetic result and no cutting, and it is the reason follow-up appointments matter.
This does not stop new damage. Sun-damaged skin keeps producing new actinic keratoses, and many people repeat the treatment every one to three years. Think of it as clearing a field rather than fixing the ground it grows in.
For sebaceous hyperplasia, the bumps flatten but the oil glands are still there, so they commonly return over a year or two.
There is no home version, and this is one where the distinction is genuinely important. The cream is a prescription drug, the lamps are calibrated medical devices, and the 48-hour light-sensitivity window is dangerous without instruction.
LED masks and red light panels sold for home use are not photodynamic therapy. They deliver light without the light-sensitising drug, which is the part that does the work. They do not treat actinic keratoses or skin cancer, and using one instead of getting treatment is how a treatable lesion becomes a larger problem.
The closest home alternatives are the prescription field creams — 5-fluorouracil or imiquimod — applied at home over several weeks. They treat the same field of sun damage and work at least as well for actinic keratoses. The trade is duration: instead of one hard week, you get several weeks of a red, raw, uncomfortable face. Some people prefer that, some strongly prefer the single session. It is a reasonable conversation to have with your dermatologist.
Daylight photodynamic therapy still requires the prescription cream and a clinic appointment, even though the light source is the sun. It is not something to attempt yourself.