A complicated new clinical trial could give some hope to Alzheimer’s patients and investors in drug companies. But there are lots of devilish details.

Drugmakers Biogen and Eisai said the top dose of an experimental Alzheimer’s drug slowed deadly brain-destroying disease over the course of 18 months in patients in the milder, early stages of the disease, reversing an earlier result from the same study that showed the drug had no benefit at 12 months. Full data were not released, but will be unveiled at a future medical meeting.