The first time one of my best friends slept with a new guy she was nuts for, she said that when the lights went off and he got his junk close enough for her to get hold of, she had to stop herself from blurting, “No, thanks, I don’t smoke.” Turned out his minuscule member was a deal breaker for her—she just couldn’t handle it (pun unavoidable).

Not all deal breakers are so cut-and-dried, and often it depends on the person—not just the person you’re dating, but also how insurmountable an issue the potential deal breaker is for you. Here are six common deal breaker categories, and how to figure out whether they’re nonnegotiables in your relationship.

• Physical turn-offs: These aren’t always major issues like the above-mentioned wee willy; another friend of mine categorically ruled a guy out over his feet (she wasn’t weird about feet, just found them attractive—and his gnarly toes and peeling soles creeped her out).

I once broke up with a guy after he informed me that his family’s teeth “went south on them” and dentures were in his immediate future. Color me shallow—you have to have teeth. Is the problem something that can be fixed? (Sorry, fake teeth don’t count with me.) If not, is it something you can get over and even learn to find adorable? (Watching him take his dentures out every night and looking at his naked gums? Nope.) Two “no” answers here mean you’re probably looking at a real deal breaker.

• Major behavioral turn-offs: Is he a chintzy tipper? Does he smack-talk his exes, his friends, his family? Is his laugh like the sound souls make when they’re sucked into the bowels of hell? Does he flirt with anything with a uterus, even if you’re right next to him? Sure, you could probably address some of these issues (e.g., “Let’s just enjoy this stand-up comedian in silence”), but are you looking for a prospect or a project?

And some of these behaviors say something deeper about someone—a person who bad-mouths alleged friends behind their backs is revealing something about her loyalty and sincerity. And you can bet a guy who can’t stop bashing his ex still has some pretty strong feelings for her.

• Lifestyle turn-offs: I used to think drug use was a deal breaker for me, until I fell for a guy who enjoyed the occasional doob. I figured it was something I could live with after all—till I found the enormous tub of weed stashed in the back of his fridge. Turned out he liked to sell it even more than smoke it. For me, drug dealer = deal breaker. For some their nonnegotiable is smoking; or alcohol or drug use (or abuse…or as in my case, trafficking). For some it’s sexual habits—a woman I know was approached by her husband several years into the marriage about whether she’d consider an open relationship. Though her initial reaction was a categorical no, he persuaded her to try it, and now they happily share the love.

Only you know what you can handle and what you’ll never be able to get past. If someone is dedicated to lifestyle choices you know are firm deal breakers for you, don’t even try to overcome them; just cut your losses.

• Core traits: In my shallow youth I went out with a man so beautiful that I regularly wanted to go places where we might be photographed together, just so everyone could see this beefcake on my arm. He was also sweet, gentle, and kind—but unfortunately dumb as a bag of hair. As much as I genuinely liked him (and liked looking at him), ultimately I knew things were going nowhere, because one of my ironclad deal breakers is intelligence.

Core personality traits—humor, sincerity, ideology, energy level, introvert/extrovert, laziness, negativity, etc.—can’t really be changed. If you’re contemplating a relationship with someone whose political leanings make you stabby, consider how it will feel after ten, twenty, or fifty years. The courtship phase is when everything is seen through candy-colored glasses. Clashing core values are never going to seem more charming as time goes on.

• Cheating: This one gets a category all its own, because it’s the biggest dividing line of deal breakers. Some women (and men) can forgive, even if they don’t forget (a certain presidential candidate—or twelve—comes to mind). For others, there’s no going back (as Robin Thicke will sorrowfully tell you). This is one you can’t really know till it happens to you—but if you’re dating someone who’s in another relationship, remember the wise words of my mother: If he’ll cheat with you, he’ll probably cheat on you. If you already know you’re dating someone with fidelity issues, is it really worth taking the chance that it’s a one-off?

• Abuse: Abuse of any sort—physical, emotional—is a deal breaker. It’s not always easy to get out of an abusive relationship, but if you’re in one, or are getting into one with someone who’s showing the warning signs [http://www.abuseandrelationships.org/...], get wise, get help—but get out.

Have you ever overcome a deal breaker—or tried and failed? What are your nonegotiable deal breakers?