Could diet pills have severe side effects when mixed with alcohol?

Diet pills +alcohol. Absolutely yes. Careful as this is a dangerous mix.
Diet pills. It depends on the type of diet pills. Some are in the category of stimulants, which suppress one's appetite. Therefore, one may be able to drink more alcohol without feeling sleepy, for example. This may lead to a dangerous situation, like trying to drive while under the influence. As such, i would be very careful. A single glass of wine or one beer is okay, but i would stop there. Good luck.