San Francisco has a methamphetamine problem. It’s bad.

I know because many San Franciscan meth users are coming to the dental school where I teach with broken, infected and missing teeth. I’m concerned.

Your mental image of a meth addict may be of a sunken-faced, wild-eyed street person with only a few remaining brown stumps of teeth. That picture is accurate for someone in the advanced stages of addiction. By the time we see these patients, few teeth can be saved.

But it may surprise you when you learn of a loved one, a teenager or twentysomething who is partying hard, just had a bad dental checkup that found six new cavities at the gumline and between front teeth. That is how the destructive dental effects of methamphetamine begin. Parents might not realize there is a connection between new cavities and recreational drug use.

Television viewers had a long and rewarding experience with the brilliant series “Breaking Bad.” Its central character, Walter White, dealt with the financial ravages of cancer by synthesizing and dealing in methamphetamine.

The world portrayed by the show’s characters included a swath of nefarious plots: criminal networking, money laundering, prostitution, murder and retribution. As terrifying as White’s conundrums were, they held the audience spellbound, waiting for the next plot twist to more fully reveal the underworld of methamphetamine. Yet the show never completely depicted the horror that methamphetamine wreaks upon its users’ health.

Meth users smoke, snort, inject or swallow pills (Yaba) of methamphetamine to experience the rush or “flash” that helps them feel more powerful, more in-control, less anxious about their lives. Meth devotees quickly experience a loss of inhibition as well as of judgment. And their numbers are growing.

More than 12 million people in the United States have tried methamphetamine at least once (4.7 percent of the population). Methamphetamine — also called speed, ice, glass or crystal — can be produced in small “labs” using such inexpensive ingredients as over-the-counter cold medications, battery acid, antifreeze, drain cleaner, lye or lantern fuel.

The toll of long-term meth abuse is startling: memory loss, aggression, mood disturbances, cardiovascular changes, malnutrition, auditory hallucinations and delusions. It devastates lives and families. What is less apparent is the effect of methamphetamine on the teeth.

Dentists can recognize a distinctive and often severe pattern of decay that seems to spread through a mouth like wildfire. In combination with clenching and grinding of the jaws, as well as a 12-hour high that causes a craving for sugary foods, the tooth decay meth unleashes is nearly impossible to reverse.

The evidence is in: Last month’s cover story in the Journal of the American Dental Association explains how methamphetamine damages teeth. Researchers began studying the mechanisms behind this rapid dental destruction and found that even a year after quitting meth, the user’s saliva remains acidic. When paired with poor oral hygiene, this biochemical phenomenon exerts a permanent effect on teeth and health.

In the not-so-recent past, full dentures were the hallmarks of the elderly. Today, too many patients in their 20s are undergoing full-mouth dental extractions and living with dentures. Dentists mostly feel that life with teeth is better than life without them.

What I need to say to you is this: If someone in your family, or circle of friends, is struggling with a meth addiction, get them help as soon as you can. The addiction is fierce, and it is intertwined with activities that are difficult to stop.

If you suspect that your loved one may be using, there are many support programs. Seek one out, because your loved one will need a program and your help to quit. Once those measures are in place, please get your loved one to a dentist — the sooner, the better.

Lola Giusti, DDS, is an associate professor at the University of the Pacific’s Arthur A. Dugoni School of Dentistry in San Francisco and a fellow at the American College of Dentists.