How long after a hair transplant can you begin trading isoretonin (accutane)?

Always follow up and ask your doctor before starting any medications.  One side effect of accutane is hair loss, so your doctor should be aware that you are starting the medication.  There is no rule when you can or cannot start accutane.

If you have achieved quite full growth at 8 months it is fine to restart the drug at month 9.  Some individuals experience full growth following their hair transplant a bit later - at around 12 months. If you feel you fit in this second category waiting a full year would be  advisable. Isotretinoin (Accutane) can impair normal healing and its responses can differ in different people. 

Best to wait a full year, but some people say 8 months is enough. Accutane can kill the old and the new hair

You should not have any complications from taking Accutane 8 months after hair transplantation. Having said that, consult with your transplant surgeon.