Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine. Alternatively, the medicine may be taken 30 minutes to 4 hours before sexual intercourse. The products discussed in this site may have different product labeling in different countries. Take sildenafil as needed before sexual activity. Ask your pharmacist or doctor for a copy of the manufacturer’s information for the patient. Please expect up to 4 to 6 weeks for reimbursement. Proper Use Use sildenafil exactly as directed by your doctor.Cialis lasts up to 36 hours in the body, while Viagra lasts 4–6 hours in the body. Both are taken 30 minutes before sex. Cialis can cause limb pain, while Viagra can cause vision change, rash. Examples of Viagra’s more commonly reported side effects include: headache. flushing (temporary warmth, redness, or deepening of skin color) stuffy nose. indigestion (upset stomach) mild vision.  · You’ll typically take Viagra about 1 hour before sex. You can take the drug 30 minutes to 4 hours before you anticipate sexual activity. Based on how well you respond to this dosage and any side.