If you use the site search with the term OD, there's tons of threads and comments that answer this.
The short answer is that most medications are designed so that they will not be fatal. It used to be that barbiturates were widely prescribed but could be fatal, and they were replaced by benzodiazepines that are not, unless mixed with alcohol and other medications. Most antidepressants aren't fatal except for Wellbutrin, which is not reliable for ctb. Medications usually have to be mixed in certain combinations to be fatal, hence the four-drug cocktail in the PPH, and none of them are easy to obtain, usually someone will only have a prescription for one or two, if that. Chloroquine is fatal but painful, so a high dose of diazepam is recommended to knock someone out before the medicine takes effect. Propanolol can be fatal at 8g or higher but is not guaranteed, and my understanding is it's more likely to be fatal when combined with a calcium channel blocker.