Before you begin using a medication, be sure to inform  your doctor of any medical conditions or Allergies you may have, any  medications you are taking, whether you are pregnant or breast-feeding, and any  other significant facts about your health. These factors may affect how you  should use Viagra (Sildenafil). 
Alcohol: Consumption of  alcohol in large amounts may decrease the ability to get an erection and may  also temporarily decrease blood pressure, causing dizziness and fainting.
Blood pressure: Tell your  doctor if you are taking medications to treat high blood pressure. The  combination of these medications and Viagra (Sildenafil) may add to the blood  pressure-lowering effect of these medications. You should also let your doctor  know if you have or have had low blood pressure.
Grapefruit juice: Grapefruit  juice can increase the amount of Viagra (Sildenafil) in your bloodstream. Do not drink  grapefruit juice when taking Viagra (Sildenafil).
Hearing problems: If you  experience a sudden decrease or loss of hearing you should seek medical  attention immediately.
Heart conditions: If you  have or have had heart disease or a heart attack or stroke, you should speak to  your doctor before taking any medication for erectile dysfunction. If chest  pains, dizziness, or nausea are experienced during sex, the exertion should be  stopped and your doctor informed.
Kidney problems: If you have  poor kidney function you may need lower doses of Viagra (Sildenafil). Kidney Disease or reduced kidney function may cause Viagra (Sildenafil) to build up in the  body, causing side effects. If you have kidney problems, discuss with your  doctor how Viagra (Sildenafil) may affect your medical condition, how your medical  condition may affect the dosing and effectiveness of Viagra (Sildenafil), and  whether any special monitoring is needed.
Liver function: If you have  poor liver function you may need lower doses of Viagra (Sildenafil). Liver disease  or reduced liver function may cause Viagra (Sildenafil) to build up in the body,  causing side effects. If you have liver problems, discuss with your doctor how  Viagra (Sildenafil) may affect your medical condition, how your medical condition  may affect the dosing and effectiveness of Viagra (Sildenafil), and whether any  special monitoring is needed. 
Nitrate-containing medications: Viagra (Sildenafil) should never be taken with any medications that contain  nitrates. Blood pressure may suddenly drop to life-threatening levels if  Viagra (Sildenafil) is taken with any nitrate-containing medication (e.g., nitroglycerin  tablets, spray, or patches) or any other nitrates (e.g., isosorbide dinitrate  or amyl nitrite "poppers"). Dizziness, fainting spells, heart attack,  or stroke may be experienced.
If use of a nitrate-containing medication is considered  medically necessary, you must wait 48 hours after the last dose of Viagra (Sildenafil)  before taking a nitrate, and only under close medical supervision. 
Penis deformation: As with  other medications for erectile dysfunction, men with an anatomical deformation  of the penis such as angulation, cavernosal fibrosis, or Peyronie's  disease should be closely monitored by their doctors if they take this  medication.
Prolonged erection: If an  erection lasts longer than 4 hours (a rare occurrence), seek immediate medical  assistance. If this condition is not treated quickly, tissue damage and  permanent loss of the ability to have an erection could result.
Prostate problems: Tell your  doctor if you are taking medications, such as alpha-blockers (e.g., Doxazosin),  to treat prostate problems, as the combination of these medications with  Viagra (Sildenafil) may add to the blood pressure-lowering effect of these medications.
Sexually transmitted infections: Viagra (Sildenafil) does not protect against sexually transmitted infections  such as HIV.
Vision problems/eye conditions: If you experience a sudden loss of vision in one or both eyes or  other vision related problems, contact your doctor immediately. Vision loss may  be partial or complete. While in some cases the condition may improve over  time, it can also be permanent.
This rare condition is more likely to occur if you have  heart disease, high blood pressure, High Cholesterol, Diabetes, or eye  problems, are more than 50 years old, or if you smoke. If you have any eye  conditions, such as retinitis pigmentosa or non-arteritic anterior ischemic  optic Neuropathy (NAION), tell your doctor.
Pregnancy and breast-feeding: Viagra (Sildenafil) is not intended for use by women.
Children: Viagra (Sildenafil) is not  intended for use by children or adolescents under 18 years of age.
Seniors: If you are 65 years  of age or older, you may need a lower dose of Viagra (Sildenafil).