Quote from Scipio -
"The injections last 6 weeks they don't end before her next scheduled injection. Her injections were scheduled every 4 weeks precisely so that there was a 2 week cushion to ensure no disruption would occur. I posted statements from her doctors indicating this to prove it".
This post from Scipio does make sense. Something as serious as Haloperidol will have to be constantly strongly in a persons body if prescribed.
A period of only a few days when it is not in a body, between injections could be fatal.
On the massacre night it was impossible for a powerful drug like Haloperidol to be strong and then disappear from Sheila's body.