8 letters went to makers with products containing the stimulant DMHA.
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They're often marketed for sports performance or weight loss.
DMHA may also appear on the ingredient panel as:
1,5-Dimethylhexylamine
1,5-DMHA
2-amino-5-methylheptane
2-amino-6-methylheptane
2-aminoisoheptane
2-Heptylamine, 6-methyl-
2-Isooctyl amine
2-Metil-6-amino-eptano
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6-Amino-2-methylheptane
Amidrine
Octodrine
Vaporpac
DMHA can cause nausea, heart palpitations, or increased blood pressure.
The other warning letters were for products with Phenibut, sometimes sold as a sleep aid.
Phenibut may also be sold as:
fenibut
phenigam
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PhGaba
Phenigamma
Phenygam
4-Amino-3-phenylbutanoic acid
-(aminomethyl)benzenepropanoic acid
beta-(Aminomethyl)hydrocinnamic acid
-phenyl--aminobutyric acid
The FDA has given the companies 15 business days from receiving the letter to inform the agency on what they'll do to comply.
The FDA can take enforcement action without further notice if the companies don't immediately cease distribution of the products.