What is the onset of action for oxygen therapy?

• A. Immediate
• B. 30 seconds
• C. 5 minutes
• D. 10 minutes

Answer: (A)

Oxygen therapy has an immediate onset because it directly changes gas exchange as soon as the inspired gas reaches the alveoli. When you deliver supplemental oxygen, the alveolar PO2 rises instantly, increasing the diffusion gradient into the pulmonary capillary blood. This quickly raises arterial oxygen tension and hemoglobin saturation, so tissue oxygen delivery improves within seconds to a minute. There’s no pharmacokinetic absorption delay, which is why longer onset times (seconds to minutes) don’t apply here.