This includes prescription or nonprescription over-the-counter [OTC] medicines and herbal or vitamin supplements. Trimazosin Other Interactions Certain medicines should not be used at or around the time of eating food or eating certain types of food since interactions may occur. Alternatively, the medicine may be taken 30 minutes to 4 hours before sexual intercourse. John’s wort. The amount of medicine that you take depends on the strength of the medicine. · decrease in amount of urine or the frequency of urination. decreased vision. difficulty in concentrating. dizziness or lightheadedness, especially when getting up .  · Adults 65 years of age and older—25 mg as a single dose no more than once a day, 1 hour before sexual intercourse. Alternatively, the medicine may be taken 30 .  · Currently, mg is the highest dose of sildenafil that’s approved by the FDA for treating erectile dysfunction. It is okay to take a mg dose of sildenafil, but .