Quit smoking timeline · 8 hours
8 hours after quitting smoking
Last reviewed July 2026
About 8 hours after your last cigarette, the carbon monoxide in your blood has dropped substantially and your oxygen levels are climbing back toward normal. Cigarette smoke fills your blood with carbon monoxide, which crowds out oxygen. As it clears through the first hours, your blood starts carrying oxygen the way it should again.
What's happening in your body
Carbon monoxide is one of the gases in cigarette smoke, and it binds to the same spots on your red blood cells that oxygen uses. While you smoke, a share of your blood is carrying carbon monoxide instead of oxygen, which is part of why smokers can feel short of breath. Once you stop, the carbon monoxide falls away over hours. By around the 8-hour mark it has roughly halved, and your blood oxygen is measurably higher.
What you might notice
- Cravings building. Nicotine has been leaving for hours now, and the urge to smoke tends to grow through the first day.
- Irritability or restlessness. Common early withdrawal, and a sign the nicotine is clearing.
- Not much physically, yet. The oxygen and carbon monoxide changes happen in your bloodstream, below the level you can feel. The felt benefits come later.
This is where the first real test lands
Eight hours in often means the end of a workday or the evening, a window when the habit used to fill the gaps. The recovery is running quietly in your blood; the challenge is getting through the cravings on top of it. Both are normal parts of the same first day.
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Launching August 15, 2026Common questions
What happens 8 hours after you quit smoking?
About 8 hours after your last cigarette, the carbon monoxide level in your blood has fallen significantly and oxygen levels are rising back toward normal. Carbon monoxide from cigarette smoke competes with oxygen in your blood, so as it clears, your blood carries oxygen more efficiently again.
How long does carbon monoxide stay in your body after smoking?
Carbon monoxide from smoking clears over several hours. It has roughly halved within about 8 hours of your last cigarette and returns close to normal within around 24 hours.
Why might I feel worse 8 hours after quitting?
As nicotine leaves your system through the first hours, cravings and irritability tend to build. Feeling worse at this stage is the withdrawal starting, not your body struggling with the recovery, which is happening in the background.
Sources: NHS, quit smoking · American Heart Association. General information, not medical advice.