The US FDA has a long article on "Hidden Risks of Erectile Dysfunction "Treatments" Sold Online". Many consumers perceive these products as safe because they appear to be properly labelled and promoted as all-natural alternatives to prescription drugs. However, FDA testing has revealed that some of these products actually contain sildenafil or vardenafil - the active ingredients in Viagra and Levitra respectively.
There are a few issues here. The FDA has a vested interest in protecting the pharmaceutical industry so they will do whatever they can to trash competing products. To pretend that Viagra, and similar products, are used purely for medical purposes and not as a recreational drug is to live in a fantasy world. People who have had their online Viagra purchases confiscated will look for alternatives. Keeping prices high by maintaining a monopoly only hits those people who live in the USA. So one "risk" of buying online medicines is that they will be confiscated by customs.
Having said that, there are people for whom taking Viagra can have serious unwanted side-effects and therefore they should inform themselves of what the real ingredients are in these alternative treatments. People with diabetes, high blood pressure, high cholesterol, or heart disease are often prescribed drugs containing nitrates, and men with these conditions commonly suffer from erectile dysfunction, but unfortunately sildenafil interacts with these drugs in a way that could potentially lower blood pressure to dangerous levels.
The article includes a long list of products that the FDA considers undesirable within the US market. The lesson here is that whenever buying anything online just do a bit of due diligence and background checks through search engines and forums. You may not believe everything the FDA says but be aware of what exactly it is that you're going to put in your body.