If you’ve ever turned down a happy hour invite because you’re on antibiotics, you probably know that alcohol and medication can be a bad, even dangerous combination. But when doctors say not to mix drinking with drugs, are they really talking about one beer or glass of wine?

That depends on the medication in question—and on other factors like your medical history, says Megan Rech, an emergency medicine clinical pharmacist at Loyola University Medical Center. In general, though, it’s smart to assume that even a tiny bit of booze can affect a drug’s safety and effectiveness. “Alcohol is metabolized in the liver, and a lot of drugs are metabolized via the same pathways,” Rech says. “So it has the potential to interact with a whole host of drugs, including things you might not think are related at all.”

Of course, some potential interactions are more worrisome than others. Here’s Rech’s advice on drinking when you’re taking any of these common meds.

NSAIDs

Long-term use of nonsteroidal anti-inflammatory medicines, including over-the-counter painkillers like ibuprofen (Advil) and naproxen (Aleve), can increase your risk of stomach bleeding and ulcers; adding alcohol can make these side effects even more likely. And then there’s acetaminophen (Tylenol), which has a direct effect on the liver—an organ that’s even more at risk from excessive drinking.

“If it’s just an isolated incident—you pop two Tylenol during the day for a headache and then you go to happy hour, it’s probably not going to increase your risk [of liver damage] that much,” says Rech. “But if you take pain meds every day, say for arthritis, and you also drink to excess on several occasions, that could really cause problems.”

Bottom line: If you’re otherwise healthy, having one or two drinks before or after taking an NSAID shouldn’t be the end of the world. But don’t make it a habit, and be careful not to overdose on either. 

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Sleeping pills

Alcohol makes the effects of sleeping pills—both over-the-counter and prescription varieties—stronger. And that’s not a good thing: Even one drink can cause drowsiness, dizziness, and slowed breathing when taken with another sedative. "Both work centrally in your brain to depress your respiratory center,” says Rech. Plus, alcohol can impair your judgment, making you more likely to have another round or pop another pill.

Bottom line: Don’t risk it—not even with one or two drinks. “There have been cases of overdose and fatality resulting from this combination, so you’re definitely better safe than sorry.” (The same goes for anti-anxiety medications, like Xanax, which also have sedative effects.)

Blood pressure and cholesterol medications

People who take drugs for a cardiovascular condition should be cautious about drinking alcohol, says Rech. “Medicines for hypertension work by lowering blood pressure, but alcohol can have an additive effect and make blood pressure drop too low, causing dizziness or fainting,” she says. Cholesterol medications, on the other hand, are metabolized in the liver, which can lead to liver damage and bleeding if you’re drinking frequently or excessively.

Bottom line: Talk to your doctor about what risks drinking could pose based on the specific drugs you’re taking. And no matter what medication regimen you’re on, don’t drink more than moderate amounts.

Antidepressants

Like sleeping pills, antidepressants can cause drowsiness and dizziness that can be made worse with alcohol; this can raise your risk for falls and car accidents. Alcohol can also keep your antidepressant from working as well as it should, and can make your underlying depression worse.

One type of antidepressant—monoamine oxidase inhibitors (MAOIs)—can cause heart problems and dangerously high blood pressure when combined with booze. “[MAOIs] have an enzyme that interacts with a byproduct of beer and wine and can cause serious side effects, including death,” says Rech.

Bottom line: “If your depression is well managed, then it’s probably okay to have an occasional alcoholic beverage while you’re on medication,” says Rech. (Unless you’re on MAIOs, she adds; then you should avoid alcohol completely.)

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Antibiotics

Your doctor will likely warn you about drinking when you’re prescribed antibiotics, and for good reason. Alcohol can make some of the unpleasant side effects of these drugs, like upset stomach and dizziness, worse than normal.

And mixing certain antibiotics with alcohol—specifically metronidazole, tinidazole, and trimethoprim-sulfamethoxazole—can be downright dangerous. These drugs contain enzymes that react with alcohol and can cause headache, flushing, a rapid heartbeat, nausea, and vomiting. Another family of antibiotics, known as cephalosporins, can also cause similar reactions, says Rech.

Bottom line: Don’t drink if you’re taking one of the antibiotics listed above, or for 72 hours after your last dose, says Rech. Drug interactions aren’t as likely with other antibiotics (like the commonly prescribed “Z-pack”) but it’s still smart to abstain for a few days. “If you’re on antibiotics, you have some sort of acute bacterial infection,” says Rech. “That’s reason enough to use alcohol sparingly while your body is healing.”

Birth control pills

The effectiveness of oral contraceptives (and other forms of hormonal birth control) isn’t affected by alcohol, so there’s no reason not to enjoy a drink here and there just because you’re on the Pill. In fact, the CDC recently advised women who weren't on birth control because they were trying to get pregnant to steer clear of alcohol due to the dangers of drinking in the early stages of pregnancy.

Bottom line: Raise a glass and enjoy alcohol in moderation, says Rech. (For women, that’s no more than one serving a day.) Be cautious about imbibing too much—which can not only cloud your judgment, but could also lead to vomiting up a recently taken pill.

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Allergy and cold medicines

Over-the-counter allergy remedies like Benadryl and Zyrtec contain antihistamines, a class of drug that can cause excessive drowsiness and may put you at risk if you’re driving a car or operating machinery—even more so when you’ve also had a drink or two. Even antihistamines that are advertised as non-drowsy (such as Claritin and Allegra) can have this effect in some people, says Rech, especially when paired with alcohol.

Antihistamines are also used in some cold and flu medicines, like NyQuil, and in some nighttime sleep aids like ZzzQuil. Some cold-and-flu formulas also contain acetaminophen, as well—all the more reason not to mix them with beer, wine, or liquor.

Bottom line: Don’t drink in excess when taking antihistamines. If you do choose to drink a moderate amount, do so in a safe setting, in which you don’t have to drive or otherwise put yourself at risk.