A itchy scalp can be due to a number of conditions. The most common is seborrheic dermatitis, a condition which affects one to three percent of adults. Seborrheic dermatitis is thought to be caused by an infection with a specific species of yeast (in the Malassezia family) which has important implications in terms of treatment options. The diagnosis is made clinically and is based on characteristic flaky, red, itchy lesions on the scalp. The treatment is daily application of anti-fungal shampoo (look for a shampoo with ketokonazole or selenium sulfide). It is best to leave the shampoo in your hair for a few minutes after deeply massaging it into your scalp and then thoroughly rinsing. Other ways to treat seborrheic dermatitis and to reduce symptoms include:
-avoiding excessive hair washing
-avoid irritating hair products (indeed, many times the cause of an itchy scalp is an allergic reaction to the chemicals in a patient's hair product)
-avoid excessive itching, which can excoriate the skin and prevent healing as well as leading to possibly infection with bacteria.
If symptoms do not improve after a month or so of treatment with an over the counter anti-fungal shampoo, you should see your doctor
for further treatment options (including higher potency shampoo). hope this helps!