Not at all! When it bites down and changes form, shellder begin to create a toxin that targets and dulls the nerves of its host slowbro’s tail. (It’s called a toxin, but it doesn’t have any real detrimental effects to the slowpoke line other than numbing the area of contact. In sufficient enough doses, it can even unlock psychic and intellectual potential.)
As for the inevitable bite wound, think of it less like teeth sinking into your slowpoke’s tail and more like a body piercing. Once the shellder has attached, so long as you remember to clean around its teeth every so often, it should be perfectly fine. It doesn’t bleed, and eventually, the wound heals around the shellder’s teeth.
In short, no need to worry, reader. Your slowpoke will be fine.
The Pokédex According to Bill 