Could you please describe some of the negative consequences that can occur when the blood-brain barrier is weakened?

Multiple causes: Inflammation due to infection, Multiple Sclerosis, arteritis or vasculitis can cause breakdown of the brain-blood barrier, as can brain hemorrhage and stroke. The outcomes depend on the nature of the causative process. Recommend information from AAN.com, or the Mayo Clinic site.

Let me guess: You've been reading about the blood-brain barrier (BBB) which before you never gave a second thought & suddenly you're worried & want to "strengthen" yours. The BBB is part of the vertebrate body plan. Nature's been perfecting it for 300+ million yr. "Breakdown" of the BBB occurs in certain disease states but it doesn't just get flabby. I think it's safe to find something else to worry about.