Medically necessary liquids are permitted through the checkpoint, but are subject to screening. If they are in excess of 3.4 ounces, these liquids are subject to additional screening, and should be declared when you reach the checkpoint. Associated supplies (such as syringes) are also permitted, and subject to screening.
If you have an external medical device, such as a port or feeding tube, let the security officer know about it when you reach the checkpoint. They will work with you to screen you appropriately, and to apply additional screening to the device itself while getting you through the checkpoint and headed toward your flight as quickly as possible.
If you have an internal medical device, such as a pacemaker, make sure you notify the security officer prior to your screening. The officer can then make sure you're screened in the most appropriate way, to help you proceed smoothly to the boarding area.
Additional information on a number of specific conditions is available on this TSA page. (Link redirects to TSA website.)