Eating grapefruit or drinking grapefruit juice when taking Viagra will lessen the effect. I didn’t realize this it and it stoped working all together.
Are you sure this wasn’t the other way around?
Grapefruit has some compunds which inhibit the enzyme CYP3A4. This is one of the enzymes responsible for breakdown and elimination of various compounds in the intestines. By taking grapefruit juice it is possible to inhibit the elimination of some compounds, of which viagra is one.
I would suggest that it’s better to avoid, or at least limit the consumption of both grapefruit and viagra together if it’s likely that you’d take them on a daily basis, as repeated daily dosages could steadily build viagra to dangerous levels.
I wouldn’t panic about a one off, but it will vary for each individual, and you could unwittingly be significantly incresing the half life of this, and other drugs you are taking.