Drop Caps Problem

I’m using thesis 1.8 and I added some code (copy and paste) to the custom functions file for drop caps. This code drops caps on every post or page, which is what I want. However, I don’t want drop caps on teasers, which contain only a few lines of text and there’s no need for drop caps on those. Can you please add something to the code below to switch off drop caps for teasers?

.custom .format_text p:first-child:first-letter {
font-size: 3.571em;
line-height: 0.76em;
padding: 0.04em 0.12em 0 0;
color: #6F6F6F;
float: left;
}