An EEG is a recording of the electrical activity of the brain. This process cannot create any side effects as your brain is making this electrical activity all the time regardless. The technician may ask you to perform hyperventilation during the test which can make you feel a bit light headed during the process as you are sending extra oxygen to your brain at this time from the deep breathing. You may also feel some numbness of tingling in your hands of feet for the same reason but this is also a normal response to the hyperventilation. It is done in EEG to help bring out baseline abnormalities in potential epilepsy sufferers. It may trigger brief episodes in petite mal sufferers that they will probably not even be aware of. Photic stimulation (strobe light) is also often used to check for a photoparyxismal or photoconvulsive response but these are quite rare even in a group of epilepsy sufferers. The light may disturb the visual cortex somewhat but this is also expected in a normal healthy brain. The combination of hyperventilation followed by photic stimulation will often cause drowsiness also, but this is also a normal response to these provocations.

I am reading a lot of confounding information on this forum about EEG causing bad effects. Please consider anxiety/stress levels, amount of sleep, other health issues, how soon after a loss of consciousness event and other factors such as recent medication changes prior to testing as the EEG is probably not the cause.