is it legal to summarize a medical journal article and sell it

I think it would be in your best interest to get a copyright license in this situation. A medical journal article is authored by someone based on their medical research. Those medical journals are published and copyrighted. Just summaries of this journal article without the express permission and no original expression (not even a derivate original expression) is not recommended. If you create an app either for profit or free without intellectual property license is an infringement. Just approach the copyright owner for a license. Disclaimer: The response is general in nature and cannot be construed as legal advice. If you would like to have legal advice, then please consult any competent attorney privately to discuss your case and establish a proper attorney-client relationship. I only work through my website at https://affordabletrademarkattorney.com/