The usual place to start looking for clinical articles is in PubMed. For complex clinical searches, use the PICO format to generate useful search terms and their relationships. See the example below to help format your own PICO search.
Patient, population and/or problem
COPD (chronic obstructive pulmonary disease)
nicotine replacement products:
nicotine transdermal patch
active control or placebo
(COPD OR chronic obstructive pulmonary disease) AND (nicotine replacement products OR nicotine transdermal patch OR nasal spray OR oral inhaler OR oral gum) AND (active control OR placebo) AND smoking cessation