It works for up to 36 hours. You can take this medicine with or without food 30 minutes before sex, depending on the dose. This medicine also works best when you take it an hour before sex on an empty stomach. Place the tip of the syringe into you or your child’s mouth and point it towards the cheek. You could also ask your pharmacist for advice on other ways to help you remember to take your medicine. This includes prescription or nonprescription over-the-counter [OTC] medicines and herbal or vitamin supplements. Manufacturer advises reduce dose to 10 mg once daily with concurrent use of some potent inhibitors of CYP3A4 avoid with ketoconazole, itraconazole and ritonavir.Sildenafil is used to treat erectile dysfunction (ED) and to reduce blood pressure in pulmonary arterial hypertension (PAH). It is taken when needed for ED and daily for PAH. A headache, stomach upsets, and flushing are the main side effects, and it should not be taken within 24 hours of a nitrate. 5. Tips. Does the price of sildenafil vary based on strength or what it’s prescribed for? 25 mg, 50 mg, mg: PAH treatment: oral tablet: 20 mg: liquid suspension: 10 mg per mL.  · Maximum dose: 20 mg orally three times a day. Injection: Initial dose: or 10 mg IV bolus three times a day. Comments: No greater efficacy was achieved with oral doses higher than the maximum recommended dose. A 10 mg injection dose is predicted to provide pharmacological effect equivalent to the 20 mg oral dose.