The FDA has sent warning letters to companies selling dietary supplements containing two ingredients that scientists say are synthetic stimulants not approved for human consumption.

The agency in April wrote to companies marketing products containing DMBA, or 1,3-dimethylbutylamine, also called AMP Citrate or 4-amino-2-methylpentane citrate, and BMPEA, Beta-methylphenethylamine, sometimes labeled as Acacia rigidula, saying the substances do not meet the definition of a dietary ingredient and any product containing them should be pulled and reformulated.

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In October, researchers at Harvard Medical School, the Netherlands Health Protection Center and NSF International found that DMBA is chemically related to the banned stimulant 1,3-dimethylamylamine, DMAA, and has not been tested in humans.

The scientists recommended that since DMBA's effectiveness and safety are unknown, it should be removed from supplements.

In 2013, FDA researchers found that the ingredient listed as Acacia rigidula in some dietary supplements was not a natural compound and, in fact, was a synthetic supplement.

But the agency did not take steps to remove the ingredient from the supply chain and would not release the names of the products its scientists tested.

In April, Harvard Medical School's Dr. Pieter Cohen, who had worked on the DMBA study, and scientists from Haverford College, the University of California San Francisco and Cambridge Health Alliance published a study identifying BMPEA in 11 of 21 bodybuilding or weight loss supplements.

According to Cohen, the ingredient was included in such quantities that consumers following the maximum recommended amount would take in more than 90 milligrams of the questionable, untested product.

"Consumers of acacia rigidula supplements may be exposed to pharmacological dosage of an amphetamine isomer that lacks evidence of safety in humans," wrote the authors, including Cohen.

After the release of that research, Sens. Dick Durbin, D-Ill., and Richard Blumenthal, D-Conn., wrote to the FDA calling for action against these "mislabeled and deceptive dietary supplements."

Agency officials gave the companies 15 business days to respond with a plan to remove the ingredients from their products or stop selling them.

Last October, military exchanges and GNC stores on military installations pulled products containing DMBA from their shelves.

The removal marked the third time in nearly three years that fitness supplements were dropped by military retailers for safety concerns.

Supplements containing the stimulant DMAA, or 1,3-dimethylamylamine, were removed in 2012 after it was found to have been used by at least two soldiers who died during physical training. The FDA issued warning letters to the manufacturer and later seized and banned products containing the ingredient after the companies refused to comply.

In 2013, the weight-loss product OxyElite Pro was removed from shelves after it was linked to numerous cases of acute hepatitis and liver failure in Hawaii, including some cases involving military dependents.

The Defense Department does not maintain a list of banned or allowed supplements; it follows FDA and DEA guidelines for what is considered illicit.

But DoD does maintain an extensive website on dietary supplements and has published a "high-risk supplement list" of products that military nutritionists and researchers say may cause troops to pop positive on a urine test because they contain undeclared drug ingredients or may be potentially harmful.

Steve Mister, president of the Council for Responsible Nutrition, a trade association of dietary supplement companies, said his organization welcomed the most recent letters from the FDA, noting that enforcement of existing laws will improve the industry as a whole.

"We're applauding the FDA in this case. Unfortunately, these kinds of synthetic stimulants give the industry a black eye. The consumer may not distinguish between a legitimate supplement and these products sold by companies on the fringe," Mister said.

Mister said acacia — a botanical supplement that comes from a tree — has legitimate uses as a dietary supplement. Historically, it has been used to treat digestion problems, coughs and dysentery. But the products cited by the FDA do not contain acacia, according to that agency's analysis.

He urged consumers to read labels carefully, look for third-party seals of approval or marks such as those from U.S. Pharmacopeia, NSF International and Underwriters' Laboratories and buy dietary supplements only from mainstream retailers — not dubious websites.

"Dietary supplements are meant to do just that, supplement the diet. They are supposed to act in ways that are subtle, just like good nutrition acts," Mister said. "If you buy something and you are getting an immediate pharmaceutical response, like dropping 20 pounds and putting on lean muscle, [you] need to be wary."

The following companies received warning letters from the FDA regarding products containing DMBA:

  • Vital Pharmaceuticals Inc. VPX Redline White Heat (strawberry, fruit punch and watermelon) and MD2 Meltdown
  • Powder City LLC. AMP Citrate
  • Prime Nutrition. PWO/STIM
  • Beta Labs. Oxyphen XR Amp’d
  • Genomyx LLC. EVOL
  • Lecheek Nutrition. Ampilean and Ampitropin
  • Iron Forged Nutrition. Contraband
  • Nutrex Research Inc. Adipodex
  • Blackstone Labs LLC. Angel Dust
  • 1ViZN LLC. Velocity
  • Core Nutritionals LLC. AMP Citrate
  • RPM Nutrition LLC. Red Rum SS
  • Brand New Energy LLC. Yellow Bullet AMP
  • DSEO LLC. HybriLean and PREAMP

The following companies received warning letters from the FDA regarding products containing BMPEA:

  • Hi-Tech Pharmaceuticals Inc. Fastin-XR (extended release), Fastin-RR (rapid release), Lipodrene (Ephedra Free)
  • Tribravus Enterprises LLC d/b/a iForce Nutrition. Conquer (Fruit Punch Slam & Raspberry Lemonade flavors)
  • Train Naked Labs LLC. Critical FX, Sudden Impact
  • Better Body Sports LLC. Phoenix Extreme
  • Human Evolution Supplements Inc. Core Burner