Are bioidentical hormones safe?

It depends what you mean. Regulated "body-identical" hormones, used in standard, prescribed HRT, are safe and well-studied. Unregulated "compounded bioidentical" hormones (cBHT) are a different thing, and are not recommended.

This is a confusing area because the marketing blurs two very different products:

• Body-identical hormones have the same molecular structure as the hormones your body makes, and modern, regulated HRT widely uses them, for example transdermal oestradiol and micronised progesterone. These are evidence-based and recommended.
• Compounded bioidentical hormones (cBHT) are custom-mixed by specialist pharmacies, often marketed as "natural" and tailored via saliva testing.

Major medical bodies advise against cBHT because it is not regulated or quality-controlled, the dosing is unreliable, saliva testing is not accurate, and safety has not been properly tested. So "bioidentical" is not automatically safer or more natural; the regulated version is excellent, while the compounded version is the one to avoid. The irony worth grasping is that the hormones many cBHT clinics charge a premium for are, molecularly, the very same ones available on a standard regulated prescription, just without the quality control, which is covered further in the benefits of HRT and weighed against its side effects.

The saliva-testing pitch deserves a specific word, because it is central to the cBHT sales model. Salivary hormone levels swing widely through the day and correlate poorly with what tissues actually receive, so "personalising" a dose from them gives a false sense of precision. Regulated HRT instead adjusts to your symptoms and standard monitoring, which is both simpler and better evidenced.

What to do: if you want body-identical hormones, you can get regulated, well-studied versions through standard prescribed HRT, so ask your GP or a menopause specialist for transdermal oestrogen and micronised progesterone, and read up on when to start HRT. Be cautious of clinics selling expensive compounded "bioidentical" hormones with saliva testing, which mainstream guidance does not support.