Abstract Background Adherence to adjuvant endocrine therapy (AET) is low in women with breast cancer. Negative beliefs about the necessity of AET and high concerns are barriers to adherence. Purpose To use the multiphase optimization strategy to optimize the content of an information leaflet intervention, to change AET beliefs. Methods We conducted an online screening experiment using a 25 factorial design to optimize the leaflet. The leaflet had five components, each with two levels: (i) diagrams about AET mechanisms (on/off); (ii) infographics displaying AET benefits (enhanced/basic); (iii) AET side effects (enhanced/basic); (iv) answers to AET concerns (on/off); (v) breast cancer survivor (patient) input: quotes and photographs (on/off). Healthy adult women (n = 1,604), recruited via a market research company, were randomized to 1 of 32 experimental conditions, which determined the levels of components received. Participants completed the Beliefs about Medicines Questionnaire before and after viewing the leaflet. Results There was a significant main effect of patient input on beliefs about medication (β = 0.063, p < .001). There was one significant synergistic two-way interaction between diagrams and benefits (β = 0.047, p = .006), and one antagonistic two-way interaction between diagrams and side effects (β = −0.029, p = .093). There was a synergistic three-way interaction between diagrams, concerns, and patient input (β = 0.029, p = .085), and an antagonistic four-way interaction between diagrams, benefits, side effects, and concerns (β = −0.038, p = .024). In a stepped approach, we screened in four components and screened out the side effects component. Conclusions The optimized leaflet did not contain enhanced AET side effect information. Factorial experiments are efficient and effective for refining the content of information leaflet interventions. Lay Summary Adjuvant endocrine therapy (AET) is a medication given to women to stop breast cancer from returning. Many women do not take AET every day or stop taking it before they should. Some women do not take AET because they do not believe it will help them, or they have concerns about the side effects. We ran an online study aiming to create the best information leaflet to help women understand how AET is helpful and to reduce their concerns. The leaflet had five sections; diagrams explaining how AET works, visual pictures of the benefits of AET, information about the side effects, answers to common concerns, and quotes from other women with breast cancer. 1,604 healthy women filled in a questionnaire before and after looking at an information leaflet about AET. Women received different combinations of the five sections of the information leaflet. We found quotes from other women with breast cancer led to more positive beliefs about AET. Some sections of the leaflet worked better in combination, while other sections were worse in combination. Our results led us to remove the detailed side effect information from the leaflet, as in combination with the other sections this negatively affected women’s beliefs about AET. Factorial experimental designs are an efficient way to optimize the content of information leaflets. Optimizing an information leaflet to support beliefs about a medication prescribed to women with breast cancer led enhanced information about side-effects to be screened out.