"We're looking at making some changes that [the letter] calls attention to," said Arbuckle, whose company is based out of Boise, Idaho. "Obviously our mission to help people with weight loss will be different. It's not going to end."
Arbuckle said he stands by his program because he says he is a testament to the weight loss success. Photos shown on his site indicate dramatic weight loss. And the testimonial he wrote on the company's website states he lost 36 pounds in 40 days.
But Arbuckle says that after being issued the FDA warning letter, he can't quite say that HCG was the source of his weight loss success.
"The program worked for me. Whether it was this or was that, I'm not sure," he said. "The program worked for me. I guess that's the safe way of stating the program."
Arbuckle declined to say how much HCG is contained in the drops and what changes will be made to the drops.
Calls made by ABC News to the other companies that were issued warning letters were not immediately returned.
It's not clear how many people are on the diet or buy these products, but Miller and Cleland said heavy marketing for these regimens on the Internet indicates there's a large consumer market out there.
Unlike companies that market homeopathic forms of HCG, some physicians like Dr. Sherri Emma, a weight loss physician based in Brick, N.J., uses the pharmaceutical grade HCG injection off-label for her weight loss patients.
Although studies show that people don't lose any additional amount of weight by adding HCG injections to their low calorie diet than with the low calorie diet alone, Emma said the difference is in how the weight is lost.
"HCG spares the muscle tissue but gets rid of the fat," said Emma, who treats about 100 patients a week with the diet program.
Emma said she's pleased about the joint agency's crackdown on homeopathic versions of HCG. Oral consumption of only trace amounts of HCG does not absorb into the body and does not work the same way as injections, she said.
In fact, HCG, a protein, is broken down as food when taken by mouth. That digestion process inactivates the hormone and therefore has no effect on the body, according to The Endocrine Society.
"I understand that this diet is becoming very popular, so I feel a sense of responsibility that people are educated and not doing things on their own," said Emma.
Still, the FDA said that regardless of the homeopathic labeling of HCG products, HCG is not an appropriate treatment for weight loss. Side effects of injection HCG include blood clots, decreased sperm production, and potential long-term risk of breast cancer in women.
About 8 million people a year fall victim to consumer fraud on the Internet, Cleland said.
Companies that have been issued a warning letter have 15 days to respond to the FDA and the FTC with steps they have taken to correct their violation or, according to the FDA website, the firms, "may face enforcement action, possible legal penalties, or criminal prosecution."