Often we wait too long before becoming proactive about hair loss. It is caused by internal factors in our body and usually not caused by topical products. As soon as you notice the loss of more than 50-150 hairs a day, it is best to see a doctor.
If advised, a doctor will order tests to see if you have any vitamin deficiencies. Sometimes severe stress is a major factor, but don't treat this by yourself. The sooner you see a professional the sooner you can begin to prevent any further loss.