Is it bad to reuse your blunt/joint filters?
Thank you for your question. The simple answer is: You can reuse your filters! They will still work, certainly. But it wouldn't be my first choice to reuse them. Some folks think that by reusing filters, they'll get an extra high from any THC that might have been left from the first joint/blunt they smoked. Even if that was the case, the amount left behind wouldn't be all that much. What you would be getting extra of, however, is all the leftover saliva and germs from everyone you shared that previous joint/blunt with. Yuck.
So, while it's not necessarily "bad," it's not ideal. And, to be honest, I'm not 100% sure why you'd even want to reuse them.
If price is an issue, you don't have to buy fancy filters (although there are now so many different types from raw hemp to even glass!). You can just use some index or note cards (or any type of card stock) that you have laying around.
Your quick question, however, does make me want to dive into an important topic: cannabis accessory hygiene. This is no longer high school. We're all adults here (in theory). There is zero reason why cannabis accessories (including filters) need to be dirty. We talk a lot about reducing the stigma surrounding cannabis. Why not reduce the gunk as well?
These days, there are plenty of products out there that can make your smoking accessories shine, so there really is no excuse for gnarly smoking gear. For instance, when I'm looking to make my bong sparkle, I use something like Formula 420, which is great for removing resin. I use it in conjunction with ResOlution Res Caps — silicone spheres that pop on the mouth piece and stem area of your bong. Toss some warm water, a little Formula 420 into the bong, cap it off, and shake it like you're Tom Cruise in Cocktail and voila! A brand new bong!
However, if you're pinching your pennies, that doesn't mean that you can't stay on top of cleanliness, as well. Sea salt and warm water also works as a mild abrasive to help clean glass. As does baking soda. Rubbing alcohol or vodka are both great for loosening and removing resin from glass, too.
Also, PLEASE CHANGE YOUR BONG WATER REGULARLY. I figure if you're asking me about reusing filters, there's a pretty decent chance that you wait until your bong water stinks like wet dog before you change it. Please don't. Clean bongwater is important for many reasons. It makes your cannabis taste and smell better. Plus, after a day, bacteria will start to build up in the still water. You know where that bacteria can end up? Your lungs.
I know as mother that I probably am a bit more of a stickler for cleanliness than your average stoner, but honestly, it really doesn't take that much. And, when you have clean accessories (or single-use filters!) everyone wins. Each puff in a sparkling bong or pipe will allow you to actually enjoy the nuanced flavors of your strain. It will also lead to less coughing and to less of a nasty taste in your mouth.
Long story short: don't reuse your filters if you want to enjoy your weed... and clean your dang bong!