Caffeine is similar to theophylline, which can be used to treat asthma through bronchodilation from smooth muscle relaxation and a reduction in airway responsiveness.

A Cochrane review found that caffeine improves lung function in people with asthma, up to 4 hours.

References

1. Welsh EJ, Bara A, Barley E, Cates CJ. Caffeine for asthma. Cochrane Database of Systematic Reviews 2010, Issue 1. Art. No.: CD001112. DOI: 10.1002/14651858.CD001112.pub2
2. DrugBank. (2005). Theophylline. [online] Available at: https://www.drugbank.ca/drugs/DB00277 [Accessed 5 Apr. 2018].