Although Viagra is often effective as a treatment for ED, some people discover that Viagra never works for them, or gradually stops working after they've used it successfully.nnThe causes of ED are complex and can include mental and physical aspects. Erections happen from and are influenced by a complicated combination of: physical mechanisms, sensations, your emotions.nnViagra may not work for you if you have: any malfunction of nerves, Peyronie's disease, anxiety, depression, stress or relationship issues.nnAlso, Viagra can lose its effectiveness over time because of changes in your body or in your health.nnIf that happens, you can try switching to another medication in the same drug class (PDE5 inhibitors).