Call your doctor right away if you have blurred vision, confusion, dizziness, faintness, or lightheadedness when getting up from a lying or sitting position suddenly, sweating, or unusual tiredness or weakness. Do not take other medicines unless they have been discussed with your doctor. Do not keep outdated medicine or medicine no longer needed. It can be dangerous to take sildenafil if you also take medicines called nitrates often given for chest pain. Use only the brand of this medicine that your doctor prescribed. If used together, your doctor may change the dose or how often you use this medicine, or give you special instructions about the use of food, alcohol, or tobacco.Read reviews and enjoy FREE shipping on most orders when you shop for sexual health products online at CVS. Browse top brand sex products today! Side Effects. 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. This medicine is available on prescription and you can also buy it from most pharmacies. It comes as standard tablets, chewable tablets, and as a liquid that you swallow. Key facts. It usually takes 30 to 60 minutes for sildenafil to work for erectile dysfunction. You can take it up to 4 hours before you want to have sex.