" There's no reason to put one after "treatment" because what follows is essential. When you set things off with a comma before and after, you are saying that it's non-essential information that simply provides more information.
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Blue-eyed SmilerIt is a treatment that slows cell destruction," he explained, "but as Jenny knows, there can be unpleasant side effects."Your comma after "explained" is right, and one belongs after "knows."
Blue-eyed SmilerI always get messed up with putting them before but's.
You can't put an apostrophe + s with "but".Blue-eyed Smiler"It is a treatment (?) that slows cell destruction," he explained, "but as Jenny knows (?) there can be unpleasant side effects."
"It is a treatment that slows cell destruction" he explained "but