A wave of unregulated peptides is sweeping the wellness world, sold online with bold promises about muscle gain, anti-ageing and fat loss. But concern is growing about side-effects, and almost nobody is asking whether these substances affect men and women in the same way. The evidence suggests they do not.

Women are one and a half to two times more likely than men to suffer a bad reaction to a drug, partly because more women take prescription medicines in the first place. They live longer than men on average and are more prone to conditions requiring long-term medication, such as osteoporosis, lupus and rheumatoid arthritis, and tend to experience more severe symptoms when they do. That raises the odds of drug interactions before biology even comes into play. Differences in how the body breaks down and clears drugs from the body, along with hormonal and immune variation, stack the risk further.

Women also tend to mount a stronger immune response to drugs, including ones that have been through rigorous testing, let alone ones that have not.

Why women’s hormones make this riskier

A woman’s menstrual cycle depends on a finely tuned communication system running between the brain and the ovaries, which keeps reproduction supplied with the right hormones. This system relies on a careful balance of several........

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