No side effects have been reported from brain entrainment use.  Most producers of the technology warn against the use of brain entrainment by children under age 18, people with seizure disorders, and pregnant women.  Studies have not been conducted on these populations to date.  Additionally, (and this goes without saying) it is suggested that you do not drive while listening to brain entrainment due to the relaxing effect.
At the beginning of May, I decided to change over to EquiSync because it’s much cheaper and let me experience the deeper levels sooner. At best, my blood pressure has been down to 66 and after cups of tea and food, back up to 76-80 which is regarded as acceptable. I was able to achieve these wonderful results in less than 6 months of daily practice of meditation using audio-beat technology.
The effects are strongest while you are listening to the tones because your brainwaves are synchronized and tuned into the frequency range you desire at that time.  After you've stopped listening the effects can still linger for a while afterwards.  The timescale will vary from person to person and be affected by what you do after you've stopped listening.
Brainwave entrainment is clearly NOT an effective standalone treatment for mental illness, but it may be an effective adjunct treatment in certain individuals. It should also be mentioned that when using brainwave entrainment, you may want to have a specific purpose in mind. If your goal is to relax, yet you’re using a protocol that is stimulating, you may falsely conclude that it doesn’t work even though you’re using the wrong protocol.
I have never been more creative, productive, more loving to all people nor have I ever been happier in my life. All the wisdom I have found in my pursuit of happiness was somewhere in my intellect. Now it is a part of me. I am using it. I now consider myself a model for younger people to let them see how it is possible to function and be everything they’ve wanted to be. I just celebrated my 85th birthday and I am as young as when I was 25. Thank you for giving me the gift of my true self.
Although most experts will tell you that people can get through the day without the production of Alpha brainwaves, that doesn’t mean there won’t be any repercussions. For people who have suppressed production of these brainwaves, they can end up developing anxiety disorders and other stress-related diseases, which can lead to the weakening of their immune system.
Since most of us spend our days in the beta state, EquiSync excludes the beta brainwave frequencies, and entrains only those responsible for the most beneficial brainwave states: alpha, theta, and delta waves. The two charts on this page illustrate the basic benefits of each brainwave state as well as the basic targeting of the EquiSync programs 1, 2, & 3.
The main gist of brainwave entrainment audios, without getting too technical here, is that they effortlessly guide one’s brain (when listened to with stereo headphones) into a specifically targeted state, as designated by the creator of the audio. They are able to alter your brainwave states through the use of specific audio frequencies, and specific beats-the most common of which being binaural beats (though there are other types of beats being used as well).  When one frequency is played into one ear- let’s say 1115 Hz- and a slightly different frequency is played into the other-say 1125 Hz- the brain is forced to reconcile the two, and creates its own “phantom” frequency that is the difference between the two- in this case 10 Hz.  Not only does this allow the brain to be led into specific brainwave states, but it also allows the two hemispheres of the brain to synchronize with each other, stimulating and promoting whole-brain functioning.
Relaxation: There is already evidence that brainwave entrainment helps improve relaxation. In studies analyzing short-term stress and anxiety, significant improvements were noted. This has been verified by other studies that suggest brainwave entrainment enhances muscle relaxation in biofeedback and can help with pain reduction and migraine headaches. The relaxation-induced response of entrainment may be responsible for a variety of the benefits.
Neural oscillations are rhythmic or repetitive electrochemical activity in the brain and central nervous system. Such oscillations can be characterized by their frequency, amplitude and phase. Neural tissue can generate oscillatory activity driven by mechanisms within individual neurons, as well as by interactions between them. They may also adjust frequency to synchronize with the periodic vibration of external acoustic or visual stimuli.[3]