Viagra Warnings 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. Medicines which interact with Viagra include: Nitrates such as nitroglycerin used to treat angina Rifampin an antibacterial used for treatment of tuberculosis Ketoconazole or itraconazole used against fungal infections Erythromycin an antibiotic Protease inhibitors such as ritonavir and saquinavir used as HIV treatments Cimetidine a drug used to treat stomach ulcers Other drugs used to treat erectile dysfunction Cialis, Levitra, Adcirca and Staxyn. How Is Viagra Supplied and Stored? How Should You Take Viagra? Liver function: If you have poor liver function you may need lower doses of Viagra Sildenafil. Medicines used to treat high blood pressure such as bosentan Alpha blockers such as doxazosin used for the treatment of prostate problems or high blood pressure. These are not all the side effects of Viagra.The medication Viagra is manufactured by Pfizer Canada Inc and it comes in 25 mg, 50 mg and mg dosage forms in a blister pack of 4 tablets. We are supplying brand Viagra . The usual recommended dose is 50 mg taken as needed, approximately 30 to 60 minutes before sexual activity. However, the medication may be taken any time from 4 hours to . Many offer prescription drugs based on answers to an on-line questionnaire. These sites tell you they will save you the “embarrassment” of talking to your doctor about certain .