An inhaler spacer is a device used to increase the ease of administering aerosolized medication from a metered-dose inhaler (MDI) . It adds space in the form of a tube or "chamber" between the mouth and canister of medication. Most spacers have a one-way valve that allows the person to inhale the medication while inhaling and exhaling normally; these are often referred to as valved holding chambers (VHC) . The spacer holds the medicine in place so that it can be breathed in easier, and it helps those who are unable to breathe deeply or synchronize their breathing with the MDI. Spacers slow down the speed of the aerosol coming from the inhaler, meaning that less of the asthma drug impacts on the back of the mouth and somewhat more may get into the lungs. Using a spacer wastes less medicine than spraying the medicine directly into the mouth, and it makes it less important to get the exact timing for activating the inhaler while taking in a breath.