Why do doctors tell you not smoke or to use caffeine before a methacholine challenge test?

For unaltered result: Since both caffeine (which can relax the bronchial muscle tone) and smoking - (which can irritate bronchial muscle tone making it more sensitive) - can affect the test - these factors and others are to be avoided ahead of a methacholine test which is trying to guage the patient's baseline bronchial reactivity (often to investigate the presence of asthma).