Some of the common side effects when using a CPAP machine include:
Skin irritation, acne, rashes, and ulcers from your CPAP mask
CPAP mask air leaks
Congestion
Dry nose, mouth, eyes, or throat
Aerophagia (air swallowing)
Discomfort when exhaling
Claustrophobia
Inability to sleep due to a noisy CPAP machine