A small percentage of animals can be sensitive to the pesticide or other ingredients used in Frontline flea medication, which can cause hair loss among other side effects.
The product is applied topically to the skin of cats and dogs in a small spot in between the shoulder blades, and sensitive pets may develop a red spot or other skin irritation, and some may develop hair loss in that area.
A general loss of hair throughout the entire body is not generally a side effect of Frontline, so if you are seeing that you will want to use seek the advice of your veterinarian.
If you are noticing hair loss on your pet around the spot where Frontline was topically applied, you may wish to try a different type of flea medications such as Advantage or Capstar to see if your pet is more tolerant to one of those medications.