The under-the-radar injections people are taking to build muscle this summer — including the 'Wolverine' shot

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• Peptide injections are soaring in popularity for fitness and longevity.
• They're marketed as a "natural" supplement for muscle-building, fat loss, recovery, and more.
• The FDA has been cracking down on peptide sellers, and experts say some caution is warranted.

People who want to feel younger, look fitter, or perhaps slough off a little layer of belly fat have been turning to an increasingly popular kind of treatment — one you can get without a prescription.