Viagra is not recommended if you are allergic to any of the components of the drug. If you experience itching, swelling of the lips and face, hives or breathing difficulty it may be a sign of an allergic reaction which requires immediate medical help. The drug is contraindicated in patients taking medications for angina (nitrates and other nitrogen oxide donators). Viagra may enhance the effect of these drugs which has a negative impact on health. Therefore patients taking angina medications should consult a doctor before taking Viagra.