Drug metabolism defines the reactions undergone by a drug when it is in the body. In general, metabolic enzymes catalyze reactions that make drugs more polar and easier to excrete. Metabolic reactions are classed as being Phase I or Phase II reactions. There are certain structures in a drug that are more prone to drug metabolism than others. Certain foods can also influence the activity of cytochrome P450 enzymes leading to the same types of problems. Inhibition or enhancement of enzyme activity will mean that the blood concentration levels of the latter drug will be different from those when the drug is taken on its own. This could lead to sub-therapeutic levels of the drug if the activity of the cytochrome P450 enzymes is enhanced, or a drug overdose if the activity is inhibited. In many cases, the functional group that undergoes the phase II reaction may have been introduced by a phase I metabolic reaction.