Quotes from link:
You Will Suffer a Lot During and After It
Being odorless and tasteless, carbon monoxide suicide is often thought to be reasonably quick, inducing swift unconsciousness and leading to a peaceful and painless death. But that’s not the real story. People trying committing suicide with carbon monoxide suffer a lot during the process. They find it hard to breathe and their head starts getting clouded. Vomiting and fainting are inevitable. Besides, fainting lowers the blood circulation rate which prolongs exposure to the gas and causes, even more damage to the body.
Complications for survivors: It is likely that you will survive the ordeal when committing suicide using carbon monoxide. When you do survive, the pain only intensifies. Depending on the degree and length of exposure, even with proper treatment, survivors may develop long-term brain damage, resulting in complications such as severe memory loss, low brain function, and severe cardiac complications. A person attempting suicide by burning charcoal will mostly survive, usually with tremendous brain damage due to cerebral anoxia.
Know more about what carbon monoxide poisoning feel like and what first aid is need here:
What Others Say About Carbon Monoxide Suicide
It is commonly believed that carbon monoxide poisoning leads to death that resembles sleep. However, that is not true. Carbon monoxide causes immense pain and discomfort and causes convulsions and muscle spasms as the body’s desperate attempts of seeking oxygen. One acquaintance of mine committed suicide using carbon monoxide and his body was so badly disfigured that his family was denied to see the body. His facial capillaries had burst, his eyeballs had popped and his tongue had swollen to prop the jaw open unnaturally. I think no one can say that is peaceful.
As far as I know, carbon monoxide is produced during incomplete combustion, which can be very difficult to produce. That might explain why it is not a very widespread method of suicide.
Suicide via carbon monoxide poisoning takes a long time and gives one a terrible headache and infinite pain. Things only get worse. If your friends, family or roommates are sleeping in the same room, the smoking of burning charcoal will cause harm to them. That guilt will swallow you! So guys, no matter how bad life appears, it gets better. Suicide should not be a way of dealing with problems, as it only hurts you, your friends and family.