While low carb diet is very popular and effective but reducing the intake of this major nutrient can cause some unwanted side effects. A typical low carb diet limits daily intake of carbohydrates by excluding or limiting wheat, bread and sweets from diet and replacing them with meats, fats, eggs, nuts, and veggies. Not of all these changes are positive, they have negative effects as well.
When you start low carb diet your body will look for sugar to burn for fuel without getting sugar from food, your blood sugar levels will decrease. With time, the body will adapt burning fat for fuel instead of protein, this turns into a process called ketosis. It is the body’s way of preserving muscle mass, which has many side effects.
Diet Doctor
•   Headache
A few days after doing a low carb diet, you will experience headaches. This happens as your brain desires running on glucose and burns the last stores of glucose before it switches to ketones for energy.
•   Bad breath
EducationWorld
When you are on the keto diet you will get bad breath as breaking down of fat releases certain chemicals that cause bad breath.
•   Disturbed menstrual cycle
Your body remains under stress most of the time when you are on a low carb diet and which at times disrupts the menstrual cycle.
•   Fatigue
WebMD
You feel extremely tired and fatigue when your body is in the state of ketosis.
 •   Constipation or diarrhea
Harvard Health
You become constipated as you skip foods rich in fiber which makes you constipated and hungry.
•   Dehydration
While you are on a ketosis state your body build up extra ketones and they can be harmful to your health. These are then passed through the body via urine and excess urination can cause dehydration.