I have cystic fibrosis but i'm wondering about trying a joint (weed) once. will it effect my lungs/health?

Not a good idea: Depending on your specific disease situation smoking a joint once may or may not affect your lungs. Having CF often leads to lung inflammation and lung damage to start with. The last thing you want to do is exacerbate those by smoking (cigarettes or marijuana).

Yes, it can: First of all, there is an old saying - try it just once and you'll try it again. Second of all, it can bring on spasms of coughing in anyone, and someone with CF would suffer far more from this happening. If you have gone 40 years without trying it, i strongly suggest going the next 40 in the same fashion.