It is a cause of various diseases.  some most common disease and other side effects caused by fast food are as follow 
Obesity- Obesity means having too much body fat
2.  H
eart disease-  High level of saturated or trans fats found in much of the food. Those fats can clog the arteries and, over time, contribute to high cholesterol levels.
3. D
iabetes- diabetes is often caused by poor lifestyle choices, such as being overweight and not being physically active. There is a side effect to consuming frequent amounts of fast food ‘obesity’ which can lead to the development of diabetes.
4. P
eptic ulcer-  the most common ulcer of an area of the gastrointestinal tract that is usually acidic and thus extremely painful. For almost 100 years, doctors believed that stress, spicy foods, and alcohol caused most ulcers. Fast Foods which may causes Ulcer are Pizzas, Chips, Salted snakes etc.