Gretchen, you could try vinegar and baking soda which works if it is not a terrible problem. We use Rid X monthly to keep drains operational.
I boil about 2 qts. water and pour it down as fast as it'll go then use the white vinegar and baking soda. Then I follow it with more boiling water with some dish detergent squirted in it. Like Sherrie says, it'll work if it's not a major problem.
Does it have a stopper in the middle? If so have you cleaned that out?
Gretchen, is it the kitchen drain or where is the drain running slowly?
Kitchen or bath? each area has their own causes...in kitchens it is normally a grease build up issue and running a lot of boiling / extra hot water can do wonders.
In the bath the slow drains are caused by hair balls 99% of the time...snaking or pulling the traps and then running the hot to clear soap residues normally get things flowing again.
I tend to run the hot on full for a few minute (10 gallons worth) every 6 months or so in my kitchen...this clears the entire line to my septic. a gallon or so only clears the traps at the sink...then the lines further down stream can clog up as the grease is cooled.
After reading most of the comments, and trying out a couple of them by myself from which I did not get the expected result. I googled and found a very handy easy to use and toxic free product, Drain-FX ( drain-fx.com ), it is a miracle product, so easy to use and has a step by step easy to use instruction on the package itself. It only took me hardly 2 minutes to connect the hose from the tap to the shower and let go the high pressurized water through the drain using your Drain-FX Plus plumbing tool, wow! it did the work the water gushed out through the drain and cleared all the clog in the drain within seconds. Believe if I had called a plumber he would have charged me at least 100 dollars for this job to get done. This is a Do It Yourself experience no chemicals or toxic materials involved totally Eco-friendly. I would recommend to anyone who is expecting a problem with their sink or has one now itself.
I have been using vinegar and baking soda for the drains for a long time but have never used the boiling water and so I got up and started the process. I am sure that this will make the difference of improving a clean "older" drain without chemicals being used.
vinegar and baking soda with hot water, I clean my drains each month with the solution