OBJECTIVES: To evaluate the effect of treatment switch on the cost-effectiveness of biologics used in patients with moderate or severe psoriasis in Colombia and Perú. METHODS: In a previous study (Alandete, JC accepted in the ISPOR 13th Anual European Congress) cost effectiveness of etanercept, adalimumab, ustekinumab and infliximab was estimated based on label information for first(induction) year and second(maintenance) year assuming a 100% treatment continuation ($1USD=COL$1.832=SOL$2.75).  For etanercept two induction schemes were considered: 50mg weekly 52 weeks-D1- and 100mg 12 weeks followed by 50 mg 40 weeks-D2-. Effectiveness was evaluated as 75% reduction in Psoriasis Area and Severity Index-PASI 75- infliximab=80%; ustekinumab=69%; adalimumab=59%; etanerceptD2-=52%; etanerceptD1=39%. Infliximab and ustekinumab effectiveness were not significantly different. Both were significantly superior to etanercept (Hawkins et al. meta-analysis presented in the 14th International ISPOR). In this abstract we developed a new model estimating switching probabilities due to treatment failure at week 12 and adverse events. Biologics costs were adjusted considering time on the pre and post-switching periods. Treatment effectiveness was adjusted when biologics were used after switching due to treatment failure. RESULTS: Introduction of switching effect ratified ustekinumab dominance in Colombia ($US44,675 in 2 years) generating cost savings of -$US4.049 versus etanerceptD1; -$US4.049 versus adalimumab; -$US7.844 versus etanerceptD2 and -$US27.517 versus infliximab; with higher or same effectiveness than the other biologics in that country. In Peru, ustekinumab changed from being the most cost-effective option and became the dominant option ($US41.827 in 2 years) generating cost savings of -$US283 versus etanerceptD1; -$US489 versus adalimumab; -$US3581 versus etanerceptD2 and -$US13.499 versus infliximab. CONCLUSIONS: In the studied countries inclusion of the switching effect due to treatment failure and adverse events ratifies cost-savings observed in Colombia and makes ustekinumab the cost-saving option in Peru. These results corroborate those observed in the USA and Europe.