Tell your doctor if sildenafil is not working well or if you are experiencing side effects. Your doctor will discuss the best treatment plan for you. Talk to your doctor about eating grapefruit and drinking grapefruit juice while taking this medicine. Figure 6. Although the interaction between other protease inhibitors and sildenafil has not been studied, their concomitant use is expected to increase sildenafil levels.Cialis is only available by prescription in the United States. Viagra: Additionally, a study from Trusted Source. found that combining L-arginine and sildenafil (Viagra) worked better for. The Savings Offer and Program expire on 12/31/ For further information, call , visit , or write: Viatris Inc., P.O. Box , Mission, KS Eligible patients may save on their VIAGRA® (sildenafil citrate) prescription with the VIAGRA savings card. Dizziness, headache, flushing, or stomach upset may occur. Vision changes such as increased sensitivity to light, blurred vision, or trouble telling blue and green colors apart may also occur. If.