I'm gonna tell ya to not re-use cottons.
I know plenty people do it and I know how you get desperate to wring out every last little drop, but it's still not a good idea.
A used cotton will get contaminated by any bacteria or fungi in the vicinity. If you're really unlucky you can fetch up with septicaemia from re-using one; especially if it's been lying around for a while.
Also, if your hands are dirty when you handle the cotton, it then carries whatever you've had on your skin. And the lovely bacterium that can cause endocarditis lives quite harmlessly on our skin... unless it gets into the bloodstream.
What I can very much recommend however is a nifty little filtering device called the sterifilt. These are designed to retain virtually none of the drug and are single - use. If you're super concerned about not wasting the least little bit these are both your most effective and safest option and can be had via mail order.