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. CONCLUSIONS: Grapefruit juice increases sildenafil bioavailability and tends to delay sildenafil absorption. Sildenafil pharmacokinetics may become less predictable with grapefruit juice. Although patients usually will not be endangered by concomitant use of grapefruit juice, it seems advisable to avoid this combination. Grapefruit juice is known to interact with some types of medications, leading to an. The special risk group are men who had heart attack or a stroke within the last 6 months. The special risk group are men who had heart attack or a stroke within the last 6 months. The clinical information is incomplete, but men who take Viagra should be aware that grapefruit juice might boost blood levels of the drug. That could be a good thing for some men with erectile dysfunction, but it could trigger headaches, flushing, or low blood pressure. History. The effect of grapefruit juice with regard to drug absorption was originally discovered in 1989. The first published clinical report on grapefruit drug interactions was in 1991 in The Lancet entitled Interactions of Citrus Juices with Felodipine and Nifedipine. Now it appears that grapefruit juice can also affect the absorption of the popular erectile dysfunction drug Viagra (Sildenafil). Because erectile dysfunction is common in some males with HIV/AIDS. If you drank a glass of grapefruit juice three days before and take Cialis, it is possible you could experience side effects related to grapefruit consumption. Therefore, if you know you may be taking the medication in the near future, it's best to avoid grapefruit products entirely. Studies have shown that if you take one viagra and drink grapefruit juice, it will be as if you took in two viagra pills. I use to drink grapefruit juice in the mornings with my pills. Drinking grapefruit juice when you take Viagra may increase the levels of the drug in your blood, putting you at increased risks of side effects, including headaches, skin flushing, and potentially dangerous changes to your blood pressure. Not only does grapefruit affect Viagra, it may also affect other medications currently taken which can further lead to drug interactions. Preventing Grapefruit-Viagra Interactions To prevent possible drug interactions and its complications, it is important to avoid grapefruit and grapefruit juice.