Who Should Not Take Peptides: A Doctor on Cancer, Pregnancy and Drug Interactions

Cancer, now or in the recent past

First, anybody with cancer, either now or in the recent past. And the reason is that a lot of peptides work by encouraging growth. They help with tissue repair, new blood vessel growth, growth hormone increases, and all that is great when you’re trying to heal from an injury. It is not great if you have cancer and we’re worried about cancer potentially spreading to other areas of the body.

Why TB-500 gets extra caution

TB-500 is one that people ask about the most. It comes from a protein called thymosin beta-4, which helps cells move around, and it helps stimulate new blood vessels in our body, which is very good for healing and recovery. It’s also what makes us particularly careful.

Now, there’s some nuances here, because there’s not direct evidence that shows that it causes cancer or increases cancer. If there was, then obviously we wouldn’t prescribe it. So the risk is, to some degree, theoretical, but we also are going to be extraordinarily cautious. Until we have safety data that definitively says this is okay, then we’re going to screen out anybody who it potentially might not be okay in. Essentially, we’re not willing to put your health at risk and take any chances.

The concern comes from animal work: in a 2003 mouse study, raising thymosin beta-4 in melanoma cells increased tumor blood vessels and lung metastases.^[1] More in what is TB-500.

Growth hormone secretagogues

Growth hormone secretagogues fall in that same category. They can be great for a lot of people, but there are groups that they’re not good for. And it’s our job to know that and to screen out, just like your physician would for anything else.

You could say, I’m interested in this blood pressure medicine. But if there’s a reason it’s not a good option for you, or you don’t need it, then you want your provider to be able to say, no, and here’s why.

Pregnancy and breastfeeding: a flat no

Second group, pregnancy and breastfeeding. And the answer there is pretty much just flat no. We don’t have safety data, and that is not something that you experiment with.

Tirzepatide and birth control pills

Somewhat related note for women: if you might be taking a GLP-1, we need to let you know that tirzepatide can decrease the effectiveness of birth control pills, especially when you first start them, or anytime there’s a dose increase. Part of the way it works is slowing down gastric emptying, which is a good thing for satiety and for making you feel full, but can be a bad thing for absorption of some medications.^[2]

The Zepbound label advises switching to a non-oral method, or adding a barrier method, for 4 weeks after starting and for 4 weeks after each dose increase.^[2]

Medications: methylene blue and SSRIs

Third group of people that we think long and hard about is people on medications that may have an impact. One of the most prominent things that comes up is people wanting to talk about methylene blue, which I think is a great option. However, it is a fairly strong monoamine oxidase inhibitor.^[3] Now, that’s not a common term that a lot of people are going to know, but again, it’s our job to know what that is and what that means.

What that means for you is that if you are on an SSRI, really quite a lot of psychiatric medications, if you’re on any serotonergic medication, then you could theoretically end up with serotonin syndrome. Now, serotonin syndrome has different levels of severity. In its mildest case, it can cause some GI distress. In its more serious instance, it can cause blood pressure and heart rate elevations, and it can actually be quite dangerous.^[3]

So if you’re on an antidepressant, that has to come up with your provider. And it will with ours. We’re going to screen, we’re going to talk to you about the risks. We’re going to tell you why you’re not a candidate for methylene blue, if you’re not.

G6PD deficiency

G6PD, which is glucose-6-phosphate dehydrogenase deficiency: methylene blue can also be quite risky for people who have that. So that’s another thing we’re going to screen for.^[3]

Blood sugar, and competitive athletes

Other things we look for: anyone with blood sugar problems, with growth hormone secretagogues. These can push glucose up higher.^[4]

Competitive athletes: some of these peptides are on banned lists, so we don’t want anybody to inadvertently get banned from something.^[5]

The 2026 WADA Prohibited List names BPC-157, TB-500, CJC-1295, sermorelin, tesamorelin and ipamorelin.^[5]

The point is not to scare anybody off

The point of all this is not to scare anybody off. The overwhelming majority of people that we work with and deal with are good candidates, and we would love to help anybody that we can. But as I mentioned, it’s our job to be the filter that makes sure that it’s safe for you to do it.

So that’s why screening is important, compared to just not having any screening mechanism, which is how people get themselves in trouble. Dr. Snipes on that difference: research peptides vs prescribed peptides.

How screening works at Rx Peps Direct

Human-use peptides need a prescription and a 503A pharmacy. At Rx Peps Direct, a licensed provider reviews your history and medications for a $39 medical review per order, covering up to three peptides, and declines a peptide that is not a safe fit. If approved, a licensed U.S. 503A pharmacy prepares it pre-reconstituted and bills the medication at wholesale, in 28 states plus Washington, DC. Browse peptides, or see are peptide injections safe and peptide side effects, a doctor explains.

This article is educational and is not medical advice. Compounded peptides are not FDA approved and are used off-label.

Sources

1. Cha HJ, Jeong MJ, Kleinman HK. Role of thymosin beta4 in tumor metastasis and angiogenesis. J Natl Cancer Inst. 2003;95(22):1674-1680. doi.org/10.1093/jnci/djg100
2. Zepbound (tirzepatide) injection. Prescribing information, sections 5 and 7 (oral hormonal contraceptives). Revised 8/2026. DailyMed, U.S. National Library of Medicine. dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=487cd7e7-434c-4925-99fa-aa80b1cc776b
3. ProvayBlue (methylene blue) injection. Prescribing information: boxed warning (serotonin syndrome with serotonergic drugs), section 5.4 (hemolytic anemia, G6PD deficiency), section 7 (monoamine oxidase inhibition). Revised 2/2024. DailyMed, U.S. National Library of Medicine. dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=4f6848e5-35ed-4046-b13c-3032b5ba3232
4. Nass R, Pezzoli SS, Oliveri MC, et al. Effects of an oral ghrelin mimetic on body composition and clinical outcomes in healthy older adults: a randomized trial. Ann Intern Med. 2008;149(9):601-611. doi.org/10.7326/0003-4819-149-9-200811040-00003
5. World Anti-Doping Agency. World Anti-Doping Code International Standard: Prohibited List 2026 (S0 non-approved substances; S2 peptide hormones, growth factors). In effect 1 January 2026. www.wada-ama.org/sites/default/files/2025-09/2026list_en_final_clean_september_2025.pdf