Accutane is a form of chemotherapy. It does lower your immunity a bit. I wouldn't worry.

I would go see a GP though and see if it hasn't turned into bronchitis. That could be a reason for the persistent cough.

Normally if you have a cough this long my GP would put me on an antibiotic.

Actually, it's very common to have a cough that lasts multiple weeks after a bad cold, on or off accutane. It happened to me last year - I got a very bad cold, but after the cold went away I had a hacking cough that didn't completely disappear until after about 5 weeks. Cough medicine will do wonders, but again - it probably has nothing to do with accutane.

Here is some info on WebMD that might put your mind at ease:

http://www.webmd.com/cold-and-flu/tc/coughs-topic-overview