OK so the only non coughing save I have is from over a week ago. I went to Cachelmo's lab where you have to go underneath into the poison gas bit. My character started coughing and hasn't stopped. I've tried lots of things, went back the chamber where it started, tried to reset the thing there, it's not an active effect although I have taken cure disease potions. I've slept for days, taken a shed load of blessings as I have the shrines in my house. No effect. he even coughs under water. Anyone found a solution? it's a game breaker because sneaking is now completely out of the question and I just started with the frigging the thieves guild. Aside from breaking the game play it's downright annoying.

Ach. Should say I'm on PS4.

I think Arcadia in Whiterun has cough drops for sale. In all seriousness, I haven't seen this one before. Not sure what your options are.....

Probably only reloading a save from before it started, and hopefully OP has that save.

See if you can find a http://en.uesp.net/wiki/Skyrim:Potions#Cure_Poison potion and if that will stop your character's coughing.