The unregulated nature of the industry in Australia and overseas means athletes can never be sure exactly what they are taking, says Nicki Vance, an anti-doping consultant who helped set-up the first drug testing programs in the AFL and the NRL in the late 1980s.
"Some medical people do believe in supplements. They are looking for any edge possible but the fact is, in most cases, we simply don't know what is in them," Vance said.
"Because the industry is not regulated, you can't guarantee what is in them.
"It can all be relatively innocent but that doesn't mean that they don't contain banned substances.
"Athletes have to be concerned when they don't know exactly what it is they are putting into their bodies, especially if it is a banned substance and they face the possibility of a significant ban.
"The fact of the matter is you don't really know exactly what you are getting.
"It is a grey line and it's the mentality that you need to take something that needs to change."
Essendon are the subject of an investigation by the Australian Sports Anti-Doping Authority (ASADA) and the league's integrity unit over unnamed supplements the club gave to players.
ASADA's involvement raises the worst-case scenario for the club and the AFL that Essendon players took substances that are banned under the World Anti-Doping Agency code.
Under that code and its notion of strict liability, players are the ones who are ultimately responsible for what they ingest, while those advising them can also be held to account.
Dr Hugh Seward, executive officer of the AFL Medical Officers' Association - the official organisation of club doctors - said players should avoid supplements at all costs.
"Many of them are not made by reputable organisations and can be contaminated with banned substances," he said.
"Players need to be very careful about exactly what it is they are taking and, with many of those supplements - whether from here or overseas - we simply do not know exactly what is in them."