Final answer:
The HRT study was halted as evidence emerged that participants on HRT had a higher risk of severe side effects such as stroke, heart attack, various cancers, and potentially dementia or depression.
Step-by-step explanation:
The study of Hormone Replacement Therapy (HRT)'s effects was halted due to evidence that participants receiving HRT faced an increased risk of various negative side effects. These side effects included an increased risk of stroke or heart attack, blood clots, breast cancer, ovarian cancer, endometrial cancer, gall bladder disease, and possibly dementia or depression.
This halting of the study highlights the complex balance between HRT's benefits, like alleviating menopause symptoms, and its significant risks, which are especially relevant to doctors when considering prescribing HRT.