PDT, or photodynamic therapy, is a skin treatment that uses a light-sensitive medication and a specific wavelength of light to destroy abnormal or precancerous skin cells. It has been FDA-approved since 1999 for treating precancerous spots called actinic keratoses on the face and scalp, and it’s also widely used for superficial skin cancers. The treatment works by applying a topical cream or solution, waiting for it to absorb into damaged cells, then shining a targeted light to trigger a chemical reaction that kills those cells while largely sparing healthy tissue.
How the Treatment Works
PDT relies on three ingredients working together: a photosensitizing agent, oxygen already present in your skin, and light at the right wavelength. Your dermatologist applies a topical photosensitizer to the treatment area. The most common options are aminolevulinic acid (sold as Levulan) and methyl aminolevulinate (sold as Metvix). These aren’t the active cell-killing agents themselves. Instead, abnormal and rapidly dividing cells absorb the medication and convert it through a natural metabolic pathway into a compound called protoporphyrin IX.
This is where selectivity comes in. Damaged and precancerous cells take up far more of the photosensitizer than normal cells do, so they accumulate higher concentrations of the light-reactive compound. When the dermatologist then exposes the area to the activating light, protoporphyrin IX reacts with oxygen in the tissue to produce highly reactive molecules, including singlet oxygen and free radicals. These molecules destroy the cells from the inside, causing the targeted tissue to break down while surrounding healthy skin stays relatively intact.
What PDT Treats
The only FDA-approved use in the United States is for nonhyperkeratotic actinic keratoses (thin, non-crusty precancerous patches) on the face and scalp. In the European Union, the approved indications are broader and include superficial basal cell carcinoma and Bowen disease, which is squamous cell carcinoma that hasn’t yet invaded deeper skin layers. These are the three conditions where PDT has the strongest evidence, and it’s considered either a first-line or well-established alternative treatment for all three.
Beyond approved uses, dermatologists also employ PDT off-label for acne, photorejuvenation (improving sun-damaged skin texture and tone), and hidradenitis suppurativa. The results for these conditions are less standardized, but PDT’s ability to target overactive or abnormal cells makes it an appealing option when conventional treatments fall short.
Clearance Rates and Cosmetic Results
For actinic keratoses, PDT performs well. FDA trials showed clearance rates of 85% to 90% after one or two sessions. More recent studies report even higher numbers: 91% complete lesion clearance at three months following one or two sessions with red light, and up to 97% clearance of individual lesions after two treatments. Cosmetic outcomes are rated above 90% for patient satisfaction.
One of PDT’s biggest advantages over alternatives like cryotherapy (freezing) is how the skin looks afterward. Cryotherapy causes hypopigmentation, or permanent lightening of the treated skin, in roughly 31% of cases. PDT causes this in 0% to 3% of cases. For people treating visible areas like the face and scalp, that difference matters considerably.
Blue Light vs. Red Light
Two main wavelengths are used in PDT. Blue light, at around 400 nanometers, has the strongest absorption by the photosensitizer, meaning it activates the chemical reaction very efficiently. However, blue light doesn’t penetrate deeply into skin, so it works best for surface-level conditions like thin actinic keratoses. This is the light source specified in the FDA approval.
Red light, at around 635 nanometers, penetrates deeper into tissue but is absorbed less efficiently, so treatment sessions tend to run longer. Dermatologists typically choose red light for thicker lesions, superficial basal cell carcinomas, or Bowen disease where the abnormal cells sit slightly deeper. Some newer devices combine both wavelengths.
What the Procedure Feels Like
A typical PDT session has three phases. First, your dermatologist cleans the treatment area and applies the photosensitizing agent. Then you wait during an incubation period, which can range from as short as 10 minutes to several hours depending on the protocol, the product used, and the condition being treated. During this time the medication absorbs into the target cells. Finally, the light source is positioned over the area and turned on.
The light activation phase is the part most patients notice. It commonly lasts anywhere from about one hour in shorter protocols to two and a half hours with lower-intensity light sources. During illumination, many people feel stinging, burning, or prickling sensations that can range from mild to quite uncomfortable. The pain tends to peak during the light exposure and fades relatively quickly once the light is turned off.
Dermatologists have several tools to manage discomfort. Cooling is the most effective topical approach. A high-airflow cooling fan or cold water spray directed at the treatment site during illumination significantly reduces pain for most patients. For people who find cooling insufficient, injectable local anesthetics or nerve blocks are options, though these are reserved for more difficult cases. Some clinics also use interrupted illumination, briefly pausing the light to give patients breaks.
Recovery and Aftercare
After PDT, the treated skin typically turns red, swells, and may crust or peel over the following days, similar to a moderate sunburn. This is a normal part of the healing process as the destroyed cells slough off and new skin forms underneath.
The most important rule after treatment is strict light avoidance, especially during the first 48 hours. The photosensitizer remains active in your skin for a period after the session, so exposure to sunlight or even bright indoor light can trigger additional, uncontrolled reactions. During this window, you should stay indoors, avoid direct or reflected sunlight (including near windows, at the beach, or in cars), wear sun-protective clothing and hats if you must go outside, and steer clear of strong reading lamps or examination lights. Even helmet-style hair dryers should be avoided. After the initial 48 hours, your dermatologist will let you know when you can gradually resume normal light exposure, though sun protection remains important throughout healing.
Most people see the treated area heal within one to two weeks, leaving smoother, more even-toned skin underneath. Depending on the extent of the condition, your dermatologist may recommend a second session several weeks later to catch any remaining abnormal cells.