Why do doctors tell you not smoke or to use caffine before the 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).