Does a Maximum Dose for Phototherapy Exist?
When prescribing narrowband UVB phototherapy, one of the most frequently asked clinical questions is:
Is there a maximum safe or effective dose for patients?
The answer is more complex than a simple number.
Understanding Dose Limits in Phototherapy
Over time, patients undergoing UVB phototherapy naturally adapt to treatment—a process called photoadaptation. As treatment progresses, the skin’s tolerance to UVB increases due to several biological mechanisms, such as:
- Thickening of the outer skin layer (stratum corneum)
- Upregulation of DNA repair enzymes
- Increased pigmentation (melanin)
This is similar to tachyphylaxis, where repeated exposure leads to reduced response at the same dose. As a result, the dose must be gradually increased throughout the treatment course to remain effective and safe.
Why a Fixed Maximum Dose Doesn’t Work
Many phototherapy protocols refer to Fitzpatrick skin type when determining starting doses. However, there’s no reliable maximum dose tied to skin type — the variation among patients within each type is too broad.
Instead, one of the most clinically relevant ways to evaluate dose tolerance is by looking at a patient’s Adaptation Factor (AF).
What is the Adaptation Factor?
The Adaptation Factor (AF) is defined as:
AF = Final Tolerated Dose / Initial Minimal Erythemal Dose (MED)
- MED is the smallest UVB dose that causes mild redness 24 hours after exposure.
- AF shows how much more light a patient can tolerate after a course of treatment compared to their baseline.
What the Research Says
A 2014 study published in the British Journal of Dermatology found that:
- AF ranged from 1.1 to 6.0 after 20 treatments
- Average AF = 2.7
- Highest variation occurred in Fitzpatrick Type III patients
- Importantly, AF did not correlate with skin type, reinforcing that skin type alone isn’t enough to predict tolerance over time.
What About Broadband UVB?
Compared to narrowband UVB:
- Broadband UVB phototherapy can produce AF values as high as 17.5
- This is likely due to greater UVA content, which can stimulate more melanin production, further increasing light tolerance.
📖 Source: Taylor CR, Stern RS. Magnitude and duration of UV-B induced tolerance. Arch Dermatol. 1991;127:673.
Clinical Takeaway for Phototherapy Protocols
Because adaptation varies so much from patient to patient:
- There is no universal maximum dose
- Protocols are most useful when based on a patient’s individual MED
- Skin type-based starting doses alone are insufficient
This supports the best practice of performing an initial MED test to personalize and safely scale treatment. Photoadaptation should be monitored closely throughout therapy.
Partnering with Phothera for Smarter Dosing
Phothera devices are designed to support custom dosing protocols and track treatment progression safely. Whether you’re prescribing in-clinic or at home, our technology and clinical support help you adjust therapy based on your patients’ real-world response—not assumptions based on skin type alone.