By Jeffrey Mark, M.D.
Medically reviewed by Frances Mark, Pharm.D.
Every week, I hear the same question. “Doctor, which peptide should I take?”
It’s an understandable question. Advanced therapies have captured the public’s attention, promising everything from weight loss and healthy aging to improved energy, recovery, and vitality. Clinics advertise transformations. Social feeds overflow with before-and-after stories. The newest molecule always sounds like the missing piece. And the same therapy can transform one person and barely register in another, which is exactly why personalized medicine must come first.
The problem isn’t the therapies. The problem is where the conversation begins. Too often, medicine has become centered on treatments instead of understanding the individual receiving them.
After more than three decades in Precision Gastroenterology, Regenerative Medicine, Metabolic Medicine and Healthy Longevity, one lesson has remained constant.
The most important decision in medicine is rarely which treatment to use. It’s determining whether that treatment belongs in the first place.
Every patient is biologically unique, with different genetics, distinct medical history, unique environmental exposures, varying lifestyle, and personalized goals. That means the same therapy can produce dramatically different results from one person to another.
At The Real Gut Doctor®, we don’t begin by asking, “What treatment do you want?” We begin with a different question.
“What changed in your biology?”
Only after understanding that answer do we determine whether an advanced therapy belongs in your care. Sometimes it does. Sometimes it doesn’t. Our responsibility is not to recommend the newest treatment. Our responsibility is to recommend the right treatment for the right patient at the right time, and to recognize when another approach is more appropriate.
That is where experience matters. Clinical judgment cannot be replaced by advertising, algorithms, or social media. It comes from decades of caring for complex patients, recognizing patterns others overlook, integrating advanced diagnostics with careful listening, and understanding how the body’s systems work together.
Sometimes an advanced therapy is an important part of restoring health. Sometimes lasting improvement begins by addressing chronic inflammation, optimizing metabolic health, correcting nutritional deficiencies, restoring digestive function, improving sleep, medication stewardship, or identifying other factors that have quietly been working against the body for years.
There is no single treatment that replaces thoughtful medicine. There never will be.
Our mission has never been to help patients collect more prescriptions, supplements, or injections. Our mission is to help people preserve the health that allows them to continue building businesses, leading organizations, traveling with confidence, remaining mentally sharp, enjoying their families, leaving a legacy, and fully experiencing the life they have worked so hard to create.
Because success is measured by more than professional achievements or financial wealth. The greatest achievement is having the health to enjoy everything you’ve built. That philosophy has guided our practice for over three decades, treating up to 3 generations in the same family. It continues to guide every recommendation we make today.
Protecting the Health That Makes Every Other Achievement Possible.
Jeffrey C.G. Mark, M.D. | Frances S. Mark, Pharm.D.
Jeffrey C.G. Mark, M.D. is Founder & Medical Director. Frances S. Mark, Pharm.D. is Co-Founder and Clinical Pharmacist.
Over Three Decades of Experience in Precision Gastroenterology, Regenerative Medicine, Metabolic Medicine & Healthy Longevity. Licensed in California, Hawaii & Illinois. Telehealth Available Nationwide.
So what does understanding the patient look like when the question is a peptide?
What does “What changed in your biology?” actually mean?
It means the first visit is an investigation, not a menu.
Four things usually change quietly over years before anyone asks for a peptide: digestive function, gut-driven inflammation, nutrient status, and the medication list itself. Each one can decide whether a treatment helps you, does nothing, or causes new problems.
Digestive function comes first
Digestion is where everything you swallow gets processed: food, supplements, and medications.
Stomach acid, pancreatic enzymes, bile flow, the absorbing surface of the small intestine, and the balance of gut bacteria decide how much of anything reaches your bloodstream. A gut that absorbs poorly can make a good treatment plan look like a failed one, because what was prescribed never fully arrived. Poor absorption can defeat even correctly chosen personalized medicine.
Many powerful peptide therapies stay injectable for a reason. Stomach acid and gut enzymes chew peptides up long before they cross into the blood. Developers solved it a few times: an oral semaglutide tablet is approved for type 2 diabetes, and oral peptide trials continue. Most still require a needle.
That is why your digestive function matters: every other plan depends on it. If bloating, gas, or irregular bowels ride along with your symptoms, that deserves testing, not tolerance. Our SIBO breath test guide is a good starting point.
Inflammation that starts in the gut
Your intestinal lining is one cell layer thick, and it decides what crosses into your system.
When that barrier weakens, bacterial fragments slip through and the immune system answers. The pattern shows up far from the gut: low-grade inflammation, skin flare-ups, joint pain, brain fog, stubborn weight. The gut is also a major immune organ, which is why problems that never mention digestion often carry a digestive signature.
A 2024 narrative review of gut microbiota, intestinal permeability, and systemic inflammation found the same three appearing together across chronic disease: microbial changes, a weakened barrier, and low-grade inflammation.
This matters for personalized medicine, too: gut-driven inflammation changes how any therapy performs. Calm the source and the same plan often works better; skip it and you may be adding a powerful therapy on top of a fire.
The microbiome runs your nutrient supply
Your gut bacteria are not passengers. They produce B vitamins, folate, and vitamin K. They break down fibers your own enzymes cannot handle, feeding the cells that keep your gut lining and metabolism working.
When two people eat the same meals and one holds healthy nutrient levels while the other slides toward deficiency, the microbiome is one reason. Your bacteria also sit in the path of nearly every pill you swallow, processing the same drug differently in different gut communities. Researchers call this pharmacomicrobiomics.
Nutrient status is never just a diet question. It is digestion, absorption, microbiome, and medication history stacked together: one more reason “take this peptide and call me in six weeks” is not an evaluation.
Medication stewardship: the list you already have
Dr. Fran, our co-founder and clinical pharmacist, runs Medication Stewardship: reviewing every medication and supplement against the biology it has to work in.
Proton pump inhibitors are effective and appropriate for real acid reflux, yet among the most overused medications in the country, often continued years after the reason resolved. A JAMA study of more than 200,000 people found 65 percent higher odds of a new vitamin B12 deficiency diagnosis with two or more years of dispensed PPI therapy, stronger at higher doses. Deprescribing reviews help: one long-term-care program moved eligible residents safely off their PPI, from 28 percent to 50.
Stomach acid has a job: digesting protein and freeing B12, iron, and other nutrients. Quiet it for years, and the job gets done badly.
If you take a PPI and are weighing peptides, GLP-1 medications, or any energy or aging therapy, review your full medication list first. It is the cheapest second opinion you will ever get.
Where personalized medicine comes in
If “every patient is biologically unique” sounds like a slogan, personalized medicine is its measurable practice.
Your genes change how you process drugs: an enzyme can burn through a medication too fast, or fail to activate it at all. Pharmacogenomics reads those differences. In a randomized trial of 900 patients with high blood pressure, pharmacogenomic-guided therapy improved blood-pressure control at six months (78 percent versus 65) with fewer readmissions. In a blinded randomized depression trial, guided care showed numerically higher remission (16 versus 8 percent), though that study was too small to prove the difference.
Not “does this therapy work in general?” but “does it work for the biology of the person taking it?” Start with our review of whether functional medicine is legitimate and our guide to how functional medicine works.
If you’re considering peptides, ask these questions first
Peptides are signaling molecules, short chains of amino acids that tell cells what to do. The best-studied are approved medicines, and honest personalized medicine starts by separating the two. Much of what is sold online is not: a 2026 review of compounded semaglutide and tirzepatide found 33 unique formulations, about half with added ingredients, and little published evidence on safety.
So before any peptide conversation, ask three questions.
What changed in my biology? If no one can answer that, nothing has been evaluated yet.
What evidence supports this specific treatment for my specific goal, and is it an approved medication or something else entirely?
Who supervises the dosing, the source, and the follow-up? A waitlist at a MedSpa is not an evaluation, any more than a shopping cart is a treatment plan.
A peptide chosen before the biology is understood is a guess with a needle attached.
How The Real Gut Doctor Helps
This is what personalized medicine looks like in practice. We start where the treatment conversation should start: what changed in your biology. We examine digestive function, gut-driven inflammation, microbiome-driven nutrient status, and your full medication list. If weight, energy, aging, or recovery is the goal, we also examine what the therapy itself will depend on: metabolic health, muscle and body composition, and sleep.
Those last layers matter. If fatty liver without any alcohol shows up in labs, or a weight-loss plan has stalled on a plateau, we look at why, because those findings change the plan. Muscle mass, not the scale alone, decides whether results hold up. Unexplained fatigue on a GLP-1 medication deserves evaluation rather than endurance.
We review history, goals, medications, digestive symptoms and the existing labs before recommending an advanced therapy, not after side effects start. Locally, we also serve Palo Alto and Santa Barbara.
If that sounds like the conversation you’ve been looking for, book a discovery call at The Real Gut Doctor. We will tell you whether a peptide or another advanced therapy belongs in your plan, and what should come first if it doesn’t. No pressure, no waitlist, and no treatment before the understanding.
FAQ
Which peptide is best for weight loss?
No honest single answer, because the working question comes first: what changed in your biology? Approved GLP-1 medications have the strongest published record for weight loss, and much of what is sold online as “peptides” has little human evidence. Evaluation first in true personalized medicine; then a therapy matched to the findings.
What should a real peptide evaluation include?
History and goals, a full medication and supplement review, digestive function, inflammation, nutrient status, and metabolic markers like blood sugar patterns and body composition. Genetic or microbiome testing when it changes a decision: the minimum for real personalized medicine, not a premium upgrade. A consultation ending with a vial and no evaluation was never an evaluation.
Do gut problems really change how treatments work?
Yes. A gut that digests and absorbs poorly can under-deliver a therapy you swallow, and gut microbes metabolize many drugs and make nutrients themselves, so the same prescription behaves differently in two people. Calmer digestion gives every other treatment a fairer chance.
Can my current medications affect peptide or GLP-1 therapy?
They already can. Blood sugar medications, blood thinners, thyroid pills, and acid suppressors like PPIs all interact with new therapies through absorption, nutrient status, and physiology. The list review comes first, and it is the step most likely to show that something you already take is quietly working against your goals.
I live out of state. Can I still be evaluated?
Yes. Dr. Mark is licensed in California, Hawaii and Illinois, with telehealth available nationwide. The first step everywhere: a discovery call covering history, medications and goals, then labs that change decisions, so the therapy choice is matched to your biology, not an advertisement: personalized medicine in the right order.
References
- Drucker DJ. Advances in oral peptide therapeutics. Nat Rev Drug Discov. 2020.
- Stein Gold L, et al. Oral Peptide Therapeutics as an Emerging Treatment Modality in Immune-Mediated Inflammatory Diseases: A Narrative Review. Adv Ther. 2025.
- Belcourt J, Sapowadia A, White CM. Compounded Semaglutide and Tirzepatide Products Use Unique Formulations but Efficacy and Safety Largely Unknown. Ann Pharmacother. 2026.
- Di Tommaso N, Gasbarrini A, Ponziani FR. Intestinal Barrier in Human Health and Disease. Int J Environ Res Public Health. 2021.
- Di Vincenzo F, Del Gaudio A, Petito V, Lopetuso LR. Gut microbiota, intestinal permeability, and systemic inflammation: a narrative review. Intern Emerg Med. 2024.
- Khan I. Drugs and gut microbiome interactions: an emerging field of tailored medicine. BMC Pharmacol Toxicol. 2023.
- Bolte LA, Björk JR, Gacesa R, Weersma RK. Pharmacomicrobiomics: The Role of the Gut Microbiome in Immunomodulation and Cancer Therapy. Gastroenterology. 2025.
- Lam JR, Schneider JL, Zhao W, Corley DA. Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency. JAMA. 2013.
- Thompson W, et al. Effect of a Proton Pump Inhibitor Deprescribing Guideline on Drug Usage and Costs in Long-Term Care. J Am Med Dir Assoc. 2016.
- Xie D, et al. Preliminary Application of Antihypertensive Gene Detection in the Treatment of Hypertension. Med Sci Monit. 2026.
- Tiwari AK, et al. Clinical utility of combinatorial pharmacogenomic testing in depression: A Canadian patient- and rater-blinded, randomized, controlled trial. Transl Psychiatry. 2022.
Medical disclaimer
This article is for education only and does not replace personalized medical diagnosis or treatment. Peptide decisions, compounded products, medication changes, and deprescribing plans should always be individualized by qualified clinicians who know your history and current health. Never start, stop, or switch a prescribed medication, including a proton pump inhibitor, without your prescribing clinician and pharmacist. If you have chest pain, trouble swallowing, persistent vomiting, blood in your stool, or unintended weight loss, seek medical care promptly. Individual results may vary.
