Americans spend about $6 billion on over-the-counter (OTC) medicines each year -- lakes of cough syrup, rivers of antacids, mountains of pills designed to alleviate the symptoms of everything from headaches to sneezing attacks. Every day, 40 million Americans use some sort of nonprescription product to diagnose and treat common ailments.

People tend to think of OTC preparations as innocuous, since they aren't controlled by prescription. Yet they can cause problems: Antacids can impede the absorption of some drugs, such as the widely prescribed antibiotic tetracycline. Chronic laxative use can lead to water balance disturbances in the body. Some drugs, such as the painkillers Advil and Nuprin, newly available without a prescription, should not be given to children under 12.

The American Proprietary Association, which represents the manufacturers of nonprescription medicines, provides the following information about self-medication: The average American has one minor ache or ailment every three days. Americans handle nine out of 10 of these problems without professional help. Americans turn to OTC preparations to treat 35 percent of such temporary problems as minor pains and injuries, coughs and colds, sore throats and skin infections, acne, allergies and arthritis.

The Food and Drug Administration requires that OTCs be safe and effective, and made according to good manufacturing standards. All OTCs are labeled according to FDA standards with the product name, statement of identity, listing of active ingredients, indications of use, directions and dosage instructions, warnings and cautionary statements, drug interaction precautions, and the expiration date. The net quantity of contents and the manufacturer's name and address must appear also.

Read those labels. They're a useful reminder that OTC medicines are still drugs -- and not to be treated lightly. Remember to report all use of such drugs to your pharmacist, if he keeps a patient profile, and to your doctor as part of your medical history.

If you have questions about OTC drugs, ask your pharmacist. Or look up OTCs you use frequently in the American Pharmaceutical Association's Handbook of Nonprescription Drugs, available at most libraries.