Urine pH

Urine pH is a great influence on whether a drug is excreted quickly or slowly and in some clinical situations is manipulated to control the excretion of certain drugs from the body.

Most drugs are either weak acids or weak bases. In alkaline urine, acidic drugs are more readily ionised. In acidic urine, alkaline drugs are more readily ionised. Ionised substances (also refered to as polar) are more soluble in water so dissolve in the body fluids more readily for excretion.

This is important in a situation such as blood poisoning. The drug must be excreted rapidly from the body, so one strategy is to alter the urine pH to increase excretion. In aspirin poisoning for example, making the urine more alkaline with sodium bicarbonate increases ionisation of the salicylic acid (aspirin metabolite) therefore increasing its excretion from the body.