First: It is technically illegal. Both sildenafil and the metabolite have terminal half lives of about 4 hours. Aaron S. Customs and Border Protection CBP will generally prohibit a supply for more than 90 days to come across the border. Your doctor will tell you not to take sildenafil if you are taking street drugs containing nitrates. It is more common for shipments that do get through to be detained for several days pending FDA inspection.You can, currently, acquire Sildenafil in the U.S. for about one-tenth the cost but sildenafil is not a generic. It is the active ingredient in viagra and is sold by compound pharmacies in the U.S. such as Riggs Drugs. In Mexico, and other tourist destinations south of the border, you are flipping a coin with what is peddled for viagra. Buying Medication in Mexico or Canada Healthy Aging Feature Stories Healthy Aging Guide From the MD Archives Buying Drugs Across the Border Discount pharmacies line the borders. But do. You can bring prescription medications approved by the Food and Drug Administration (FDA) from Mexico into the United States. But there are limits. The U.S. Customs and .