There is considerable evidence that prescribed drugs are not taken as intended. We present a stochastic pharmacokinetic mathematical model that can be used to assess the implications of non-adherence to prescribed drug regimens, on the part of either patients or health professionals. In the context of an orally administered drug, explicit equations are derived for the time varying mean and variance of the concentration of the drug in the serum, depending on the probability of each scheduled administration being omitted. The analysis presented here can be used to assess whether a given level of non-adherence is likely to have a clinical impact. The methods used can easily be adapted for use in the context of other routes of drug administration.