Whatever you need, you can get it online. That can make shopping for health products a little bit, shall we say, sketchy.

“The people selling certain products to you don’t care about your health and just want money. With greed comes a lot of fraud,” said Josephine Dlugopolski-Gach, assistant professor of internal medicine and pediatrics at Loyola University Health System.

While you have to be careful with whatever you buy, these four products below can run you into a lot of trouble—and harm:

Certain prescription medications
If a site will let you buy meds without a prescription, that’s a big red flag—especially for certain medications. Listen to this warning from the Drug Enforcement Administration: “Buying online could mean doing time.” Even if you have good intentions, you can’t legally buy “controlled substances” online like Xanax or Ambien without an Rx.

And prescriptions from cyber docs won’t cut it, says the DEA. The law is different depending on your state, but most require you to see a doctor you have a relationship with in person. In addition to that, buying from a bad site could leave you with medication that’s fake or contains dangerous ingredients. For example, the FDA purchased the flu-stopping medication Tamiflu online in order to test it. They found it wasn’t Tamiflu at all, but a combination of talc and acetaminophen.

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It’s perfectly fine to buy prescription medication from a state-licensed US-based online pharmacy; these sites often can help you save money. To know if they’re legit, Dlugopolski-Gach suggested making sure they have an actual phone number, have a licensed pharmacist on staff, and require an Rx to fill your order. You can check the legitimacy of the site at the National Association of Boards of Pharmacy. And use common sense, “if the deal sounds too good to be true, you’re probably not going to get the right medication,” she said.

Weight loss supplements
You never know what you’re going to get when you buy a weight loss supplement online. The FDA points out that in their testing, they’ve discovered supplements—even herbals—were tainted with hidden and unsafe ingredients. Many are also not FDA-approved, meaning their claims haven’t been checked out and aren’t regulated. (It’s on the individual companies to tell the truth. They don’t always do that.)

“A lot of times, these weight loss pills are just stimulants. They contain a lot of caffeine, which is not safe, especially if you have a cardiac condition,” said Dlugopolski-Gach. “I’ve seen people go into the ER on the verge of a heart attack.”

While building long-term healthy habits is often the best way to keep weight off, if you want to check out something that promises to help you lose weight or rev your metabolism, “tell your doctor what you’re interested in before you buy it, even if it’s marketed as natural,” she added.

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Breast milk
You hear “breast is best”—but it’s not if it comes from an online source, suggests an editorial in The BMJ. The problem is, breast milk online is an unregulated industry, so it can be contaminated with viruses (like hepatitis or HIV), bacteria (if not stored or shipped properly) alcohol, prescription medication, and illegal drugs, noted Dlugopolski-Gach. What’s more, in a new study in the journal Pediatrics, researchers tested 102 samples and found 10 percent were topped off with cow’s milk, which can cause an allergic reaction.

It’s understandable that people might want to buy breast milk–aka “liquid gold”–if they can’t produce their own due to cancer treatment or other reasons. Or that other women would want to donate or sell their milk if they produce more than enough. However, the temptation to make more money by adding cow’s milk might be too much for some online sellers.

Organizations like Eats on Feets and Only the Breast  (which broker such sales) do recommend pasteurizing all milk and screening donors for HIV and other diseases (among other safety suggestions), but many people don’t follow the guidelines, according to a CNN report.

“If you can breastfeed that’s wonderful, but if you can’t, formula is the next best thing,” Dlugopolski-Gach said. “It’s not worth risking going to an online source and getting breast milk from a stranger.”

If you do want to donate milk, there are nonprofit milk banks that collect, test, pasteurize, and store human milk for infants, mostly at-risk neonates in hospitals; go to theHuman Milk Banking Association of North America for more information.

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Hormone products
If you are approaching menopause, you might be tempted to buy hormone replacement medications, creams, or herbs online.

“Some women want a quick fix to get their sex drive or chutzpah back,” explained Dr. Diana Bitner, an ob/gyn at Spectrum Health Medical Group in Grand Rapids, MI. “I have patients who have bought testosterone pellets on their own. They end up taking so much of the hormone they have really bad side effects, like hair growth, voices deepening, and rage issues,” she explained. Many of these products are not effective, safe, and contain variable amounts of active ingredients.

Same goes for buying soy. “Women will buy a ton of this online and say it doesn’t make them feel better, so they buy more and more,” Bitner explained. Only about 30 perceent of women’s bodies can actually utilize soy to lessen menopause symptoms, so you may be wasting your money. For any hormone treatment, even if it’s labeled “natural” you need a doctor’s guidance; she can ensure you get the right hormones in the right amount every time that work.

This article originally appeared on Health.com.