But the reason gives some health officials cause for alarm.

The rate of new HIV infections among gay men have fallen drastically in England, according to a new study. On Wednesday, a report presented at the HepHIV conference in Malta revealed diagnoses among gay men have dropped nearly a third.

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Researchers theorize it’s because people have begun taking PrEP, even though it’s not currently available on the National Health Service. (Out of pocket, a monthly course of Truvada can cost upwards of $5,000 in England.) It’s believed generic versions are being ordered from online pharmacies in India and other countries, without a prescription.

Buying unregulated medication has serious risks, a spokesperson for the UK Medicines and Healthcare products Regulatory Agency told New Scientist.

“Medicines purchased in this way could have the wrong active ingredient, no active ingredient, or an incorrect dosage. Prescription medicines are ’prescription only’ for good reason.”

The website I Want PrEP Now works with NHS clinics to ensure the drugs are authentic, and doctors run tests on users to make sure they’re not causing side effects like kidney damage.

Experts believe the drop could also be attributed to better testing and more widespread and quicker use of antiretroviral drugs by those who test positive.

Whatever the cause, this decline will have a significant impact on HIV cases overall in the UK, where more than half of new infections are among men who have sex with men.