Half life refers to the length of time the drug stays in your system.
The half life of citalopram is 36 hours, which means 36 hours after taking a tablet half the dose is still active in your body. If you take a tablet every 24 hours you have a fairly constant supply in your body and even if you miss one tablet the results are not so dramatic.
The half life of your duloxetine can be as little as 8 hours, meaning that if you miss a dose the amount of the medication in your system drops quickly and you would notice a missed dose very quickly.
does that help?