BH4 (Tetrahydrobiopterin)

Tetrahydrobiopterin · Sapropterin · 6R-BH4

Snapshot

Indications	Contraindications
BH4-responsive phenylketonuria¹ BH4 deficiencies² Dietary phenylalanine tolerance³ Inattention in phenylketonuria⁴ Endothelial function^{5, 6} Autism spectrum symptoms^{7, 8}

Tetrahydrobiopterin (BH4) is the natural enzyme cofactor for phenylalanine hydroxylase, the monoamine-synthesizing hydroxylases, and nitric oxide synthase⁹. Supplementation, usually as sapropterin, aims to restore cofactor availability where the pathway is deficient or overwhelmed.¹ Established use is BH4-responsive phenylketonuria (PKU)^{1, 10} and primary BH4 deficiencies². It is also studied for endothelial function¹¹ and for inattention and autism spectrum symptoms¹². The typical patient has PKU and has been identified as sapropterin-responsive¹.

A Cochrane review and a meta-analysis of randomized trials found short-term lower blood phenylalanine or higher dietary tolerance in responsive PKU^{1, 10}. Vascular evidence rests on small forearm infusion studies showing better endothelium-dependent vasodilation in hypertensive patients and smokers^{13, 14}. In a 2022 randomized crossover study of postmenopausal women, one oral BH4 dose raised flow-mediated dilation, but adding intravenous vitamin C did not restore premenopausal levels¹⁵. Products are sapropterin dihydrochloride, a synthetic BH4, and the precursor sepiapterin, which raised plasma BH4 more in a phase I study in healthy volunteers¹⁶; see Form comparison.

A three-year extension of a pediatric sapropterin trial reported that frequent blood phenylalanine monitoring and careful dietary titration are necessary³.

Last updated October 15, 2026. Read how we make monographs.

In the full monograph

57 sources, 50 studies

When to useThe situations practitioners use it for, each with the reason and the study behind it.
SafetyWho should avoid it, who needs extra care, and the side effects reported in studies.
InteractionsThe medicines and supplements it interacts with, the effect of each, and what labeling says to watch.
Protocol notesThe doses used in studies, how and when they were taken, and how the forms on the market compare.
How it worksWhat it is and how it works in the body, in plain terms.
Published studiesThe studies behind each use, popular claims checked against them, and trials still under way.
Word of mouthWhat patients and practitioners report online, kept separate from the clinical evidence.
Clinical perspectiveA closing read of where the evidence is solid, where it is thin, and what has not been studied.

Snapshot

In the full monograph

Sources