When buprenorphine/naloxone (Suboxone) is taken as prescribed by itself (without any other sedatives), it has a ceiling effect, meaning that taking more and more will not lead to respiratory depression (slowing or stopping breathing) that causes an overdose. Hence it is considered a very safe medication.
However, if patients take other sedatives on top of this (such as benzodiazepines - like klonopin, xanax, ativan, or valium) or drink alcohol, these other medications take away the ceiling effect. A person might then experience the additive effects of sedation and respiratory depression, leading to an overdose. You should NEVER mix sedative substances with Suboxone without consultation and careful monitoring by your doctor.