The phrase "insidious effect" means something that seems harmless or unnoticed at first, but gradually causes harm or negative consequences over time. Full definition
There is an apocalyptic tone to much of her work but she also demonstrates a concern with the more insidious effects of global warming and consumerism. (royalacademy.org.uk)
Bernard's sermonizing and Joan's all - too - blunt way with words around tricky subjects have quietly insidious effects on their children, as cursing becomes second nature and undigested thoughts are released in public to confused ears. (slantmagazine.com)
Caffeinated drinks — such as coffee, tea, soda, and energy drinks — pep you up temporarily, but they may have a more insidious effect. (mindbodygreen.com)