Patients lie. Constantly. It's frustrating enough to make a diagnosis without half the things the patient says being freaking lies. Or better yet, they intentionally omit things.
Sometimes it's due to embarrassment. Like I had one patient who didn't want to admit to me that his stroke symptoms occurred while he was having sex with his wife. That's understandable, I guess.
We assume everyone is lying. No matter what they tell us, we always get a urine tox. We always add few drinks to what the patients say their daily EtOH consumption is. Even if a woman claims she hasn't had sex in a year, we do a pregnancy test. I've heard the whole logic of starting pap smears on woman at age 21 regardless of sexual activity is that if a woman is 21 and says she's never had sex, she's probably lying.
It's sad that it's gotten to the point where if a patient asks for an excuse to stay out from work or a few vicodin, my radar goes up immediately... even if they very clearly need what they are asking for.
For example, a while ago, a quadriplegic patient asked me to fill out a disability form for him (for a few weeks) because he had fractured his leg and couldn't transfer into his car anymore to drive to work while in the cast, my automatic thought was, "Is he trying to sucker me into signing a disability form?" That's really and truly sad that I've been conditioned to think that way. It's not good to be a pushover, but I wish I weren't that skeptical.