Wikipedia claims that the
minimal amount of 15-20 grams (taken orally) of boric acid
can be lethal. Here's the exact quote. "
The minimal lethal dose of ingested boron (as boric acid) was reported to be 2–3 g in infants, 5–6 g in children, and 15–20 g in adults. [...] However, a review of 784 human poisonings with boric acid (10–88 g) reported no fatalities, with 88% of cases being asymptomatic." It doesn't sound all that reliable. Also, keep in mind that it's used as an insecticide and therefore it's meant to not be lethal to children who accidentally consume it.
Azcentral includes a potential risk of boric acid consumption or exposure is infertility in men. If you don't die, it would lead to some unwanted side effects. Vomiting also seems like a common side effect, in which case you'd need something like metoclopramide.