How does grapefruit juice affect how Viagra works? Is it safe to take them together?
There is a well-known effect where grapefruit juice can interact with medications. It may increase the "bioavailability" of Viagra, increasing its effectiveness.
In effect, what you are doing is mixing drugs, which can have unpredictable effects. When you drink the grapefruit juice (or eat the grapefruit), how much you drink or eat, what type it is, etc. can all play a factor. What else you've eaten, your weight, etc. can also have an effect. So there's just no way to predict how your body will react.
So, generally, it's not recommended to mix the two.
If you are determined to try this combination, we recommend trying it at home, in a safe environment, with a very low dose of Viagra at first - at most 25mg, and perhaps even less. That way you can test the drug interaction. You may experience better effectiveness, but you also may experience increased side effects like headache, stuffiness, etc.
Note: if you are taking any other medications, or have any other serious health issues, you should consult with your doctor before even considering this. The grapefruit juice could interact with some other medication you are taking, with truly unpredictable, and perhaps dangerous, results.