The sources provided do not provide a specific date or timeline for when skin irritation related to Cosentyx began. However, the sources do provide information on the mechanism of action of Cosentyx and the potential side effects associated with its use. Cosentyx (secukinumab) is a biologic medication that works by blocking a protein called interleukin-17A (IL-17A) that is involved in the inflammatory process in psoriasis and other autoimmune diseases [2]. Cosentyx is used to treat moderate to severe plaque psoriasis, psoriatic arthritis, and ankylosing spondylitis [2].

According to a study published in the Journal of Immunology Research, Cosentyx has been shown to be effective in treating psoriasis with a favorable safety profile [3]. However, like any medication, Cosentyx can cause side effects. The most common side effects reported with Cosentyx are upper respiratory tract infections, diarrhea, and injection site reactions [2].

Skin irritation, including rash and itching, has been reported as a potential side effect of Cosentyx [2]. In clinical trials, the incidence of skin reactions with Cosentyx was generally low and similar to placebo [2]. However, individual experiences with medication can vary, and some patients may experience skin irritation with Cosentyx.

In summary, while the sources provided do not provide a specific date or timeline for when skin irritation related to Cosentyx began, it is known that skin irritation is a potential side effect of the medication. Patients taking Cosentyx should be aware of the potential side effects and report any concerns or adverse reactions to their healthcare provider.

Sources:
[1] https://www.drugpatentwatch.com/p/biologics/tradename/COSENTYX
[2] https://www.cosentyx.com/plaque-psoriasis/how-cosentyx-works
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5013855/