Actually, I really hate the whole idea that one anti-depressant is "stronger" than another -- no, they're all different, they all work for some people and not others, and they all work a bit differently. But none of those are the same as their being "stronger."

Citalopram is different from fluoxetine. Period. They're both SSRIs, so they're both in the same class of drugs, they both inhibit the reuptake of serotonin through effects on the serotonin reuptake transportase, but they're different -- not weaker and stronger.

That said, because each SSRI is a bit different from the others, if one doesn't work, another might. It's well worth trying, but it's not nearly as simple as stronger and weaker drugs.

Good luck.