Visual snow syndrome (VSS) was officially recognized in 2025 with a unique code in the eleventh edition of the International Classification of Diseases by the World Health Organization. Very few cases of VSS have been reported with 22 individual drugs or vaccines through the Yellow Card scheme.

VSS is a rare disorder and the underlying cause currently unknown. The Medicines and Healthcare products Regulatory Agency (MHRA) keeps all licensed medicines and vaccines under close monitoring and any potential signal of an association between a medicine or vaccine exposure and development of VSS will be rigorously assessed through the MHRA’s multidisciplinary signal review process and added to individual product information when there is data available to support regulatory action.

The Medicines and Healthcare products Regulatory Agency (MHRA) is responsible for ensuring medicines, medical devices, and blood components for transfusion meet applicable standards of safety, quality, and efficacy. The MHRA rigorously assesses the available data, including from the Yellow Card scheme, and seeks advice from the Commission on Human Medicines, the MHRA’s independent advisory committee, where appropriate, to inform regulatory decisions, including amending the product information.

The MHRA has received a total of 44 United Kingdom based reports through the Yellow Card scheme associated with the reaction term visual snow syndrome received up to and including 25 November 2025. The table attached shows the yearly breakdown of substances associated with visual snow syndrome, each year from 2017 to 2025.

It is important to note that anyone can report to the MHRA’s Yellow Card scheme, and the recording of these reports in the Yellow Card database does not necessarily mean that the adverse reactions have been caused by the suspect drug. Many factors must be considered in assessing causal relationships, including temporal association, the possible contribution of concomitant medication, and the underlying disease. We encourage reporters to report suspected adverse reaction reports, and the reporter does not have to be sure of a causal association between the drug and the reactions, as a suspicion will suffice.

The number of reports received cannot be used as a basis for determining the incidence of a reaction, as neither the total number of reactions occurring, nor the number of patients using the drug, is known. All fatal reports, including those reporting completed suicide, are assessed by the MHRA and cumulative information is reviewed at regular intervals.