Information on How to QUIT Smoking Cigarettes:

How to Avoid the Side Effects Of Quitting Smoking: Concentration Loss

Dealing with Loss of Concentration after Quitting

The loss in concentration that you face when you quit smoking can also be attributed to low blood sugar. Coupled with your body now having increased oxygen levels in its blood, this makes it difficult for your body to cope with. These opposing signals to the brain can lead it to be in a state of temporary confusion, thereby causing loss in concentration.

Remember that your body would need some time to get used to living without nicotine stimulation, and this is a symptom that should not last for more than a few weeks.