Is minoxidil foam less effective than minoxidil liquid?

Although the ultimate result wouldn't be different whatever version you use, liquid Rogaine may cause slight irritation indeed.
But we also recommend the foam for another reason; you have more control over the application. Foam is better in terms of direct application on the recipient area as it doesn't spill around like liquid, so you may consume less of it per session, meaning the bottle can last for a longer period. 

Hello and that you for your great question. Personally I prefer the liquid as its easier to apply directly to the skin.