If you need emergency medical care for a heart problem, it is important that your doctor knows when you last took sildenafil. Consult your healthcare practitioner for specific guidance if you are uncertain about the right dosage or timing. However the dose is usually not more than 40 mg 3 times a day. Food and Drug Administration has approved fifteen drug manufacturers to market generic sildenafil in the United States. Be sure to let your doctor and pharmacist know that you are taking these medications before you start taking sildenafil.Sildenafil is used to treat male sexual function problems (impotence or erectile dysfunction-ED). In combination with sexual stimulation, sildenafil works by increasing blood flow to the penis to.  · 2. Rex MD – Free Viagra Samples. Rex MD is a men’s health-focused telemedicine service. They provide free prescription pharmaceutical samples for erectile dysfunction and hair loss and. Interactions. 1. How it works. Sildenafil may be used to treat erectile dysfunction (ED or impotence) in men (Viagra brand) or pulmonary arterial hypertension in both men and women (Revatio brand), depending on the brand. Sildenafil blocks the action of an enzyme known as cGMP-specific phosphodiesterase type-5 (PDE-5).