I Have A Huge Bald Spot On The Top Of

I have a huge bald spot on the top of my head from wearing wool hats for so many years. I have tried for years to find a product that will help it grow back. I was told about a product called Tricov that patients taking chemotherapy have used to regrow their hair. Are there any side effects to using this product? Or can you recommend another over the counter drug other than Rogaine that I can use on this spot.