Elev8 Featured Video

About a third of men and women in the United States admit to lying to their doctors, according to a 2010 survey.

But keeping secrets can be dangerous, especially if the information you’re withholding leads to a wrong diagnosis. Besides, “a doctor’s job is to advocate for your health, not judge,” say Gary Fischer, M.D., a general internist at the University of Pittsburgh Medical Center.

Read: How To Handle Your Pets During Your Move

So here are six untruths people often tell their doctor…versus the real situation that doctors NEED to know about.

“I never smoke.”

Actually, if you puff even the occasional cigarette at happy hour, your doctor needs to know. Yes, cancer is a concern, but equally important, those smokes affect your circulation and increase your risk for blood clots. Be especially careful if you’re on the Pill. “Estrogen, found in combination birth control pills, the Patch and the NuvaRing, makes blood more prone to clotting,” explains Mary Jane Minkin, M.D., clinical professor of obstetrics and gynecology at Yale School of Medicine.

Your doc can recommend estrogen-free contraceptives, like the progestin-only mini pull, an IUD or the Depo-Provera injection, but only if you’re up-front with her. Still on the fence about fessing up? You may not have your habit under control as well as you think: One study found that one in five social smokers eventually starts lighting up daily.

“Nothing’s changed since my last visit.”

Really? When your doctor asks “What’s new?” she doesn’t mean only medically. Whether you’re in a tense relationship or fretting about work woes or money matters, chronic stress can trigger high blood pressure, migraines, tummy troubles and heart palpitations, Dr. Fischer says.

“The stress hormone cortisol throws other hormones, like estrogen and progesterone, out of kilter,” says Jerilynn Prior, M.D., director of the Centre for Menstrual Cycle and Ovulation Research at the University of British Columbia. This does more than mess with your period: Estrogen helps prevent bone loss, and progesterone aids bone growth.

Make sure to check out what the rest of the lies are here.