The most common causes are usually one or more of the following: - namely flea allergic dermatitis, so you will need an effective flea treatment and control protocol if this is the case.
Another other common condition is food allergies, so a hypoallergenic diet trial is recommended for a 2 month trial.
There may also be an ear mite infestation which often causes intense itchiness to the extent that they cause some bad skin infections around the head and neck from the claws scratching at the head/ears.
Other causes include cat fighting and ringworm. You should really be counselled by a Veterinarian as to what the best course of action and management should be. It is always easier to treat something that you can diagnose rather than scramble for different treatments and hope one of them works.
Even if one of the above is correct e.g. fleas, often just flea treatments may not be enough, a course of cortisone and antibiotics is also needed sometimes, or medicated washes etc. to help keep this cat comfortable and treat it more effectively.
Martin Slome DVM
Centre Street Animal Hospital
7700 Bathurst Street
Tel: (905) 771-9855