spam links appear in page source code

I just discovered that when I look at the source code for a bunch of the wordpress pages (dashboard for example), there is a bunch of garbage at the beginning including links to different commercial sites. Here is a sample:

<a href="https://www.lexapro-side-effects.org" rel="follow">Lexapro Side Effects</a> <a href="https://www.couponstocks.com/walmart-coupons" rel="follow">walmart coupons</a> <a href="https://www.couponstocks.com/office-depot-coupons" rel="follow">Office Depot Coupons</a> <a href="https://www.lowes-coupons.com" rel="follow">lowes coupons</a> <a href="https://www.draw-somethingcheat.com" rel="follow">Draw Something Cheat</a>

How did these get in here and how can I get them out? They appear on one of my sites but not the other.