Do GLP-1 injections change how oral medicines are absorbed?

Why this study matters

GLP-1 medicines such as semaglutide (Wegovy, Ozempic) and liraglutide (Saxenda, Victoza) slow down how quickly the stomach empties. That is part of how they reduce appetite. But a slower stomach could also change how quickly pills are absorbed. This matters most for medicines with a narrow therapeutic index, meaning the gap between a helpful dose and a harmful one is small, and for medicines where timing matters, such as the contraceptive pill. Many Canadian readers take other medicines alongside a GLP-1 injection, so this review addresses a common question.

Who was in the study

This was a systematic review, which means the researchers searched the literature in a planned way and summarized every relevant study they found. They searched the PubMed and EMBASE databases up to November 1, 2023. They included 22 published reports and 6 product prescribing sheets. Prescribing sheets are official product documents that sometimes report interaction data from studies that were never published in full.

Most of the studies enrolled healthy volunteers rather than people living with obesity or diabetes. The summary we reviewed does not report the total number of participants or their ages.

What the researchers did

The researchers looked for pharmacokinetic studies of any injectable GLP-1 receptor agonist given together with an oral medicine. Pharmacokinetic studies measure how a drug moves through the body: how fast it is absorbed, how high its level gets, and how much of it reaches the bloodstream overall. Two authors independently extracted the data.

Three measures were the focus:

• Cmax: the peak level of the oral drug in the blood.
• Tmax: the time it takes to reach that peak.
• AUC: the total exposure to the drug over time.

The review also looked for any reported clinical endpoints, such as whether the oral drug still worked as expected. The authors did not pool the results into a single number, so this is a systematic review without a meta-analysis.

What they found

Across the different types of oral medicines studied, treatment with a GLP-1 medicine either did not affect the peak level or reduced it, and it delayed the time to peak (R32-C125). This pattern was seen with:

• warfarin, contraceptive pills and acetaminophen (drugs that dissolve and are absorbed easily)
• ACE inhibitors, a type of blood pressure medicine
• statins, used for cholesterol
• digoxin, a heart medicine

However, the GLP-1 medicines did not cause clinically significant changes in total exposure (AUC), and no differences in clinically relevant endpoints were reported. The authors say the lower and later peak levels fit with the known slowing of stomach emptying, but the overall amount of drug absorbed was not considered clinically significant.

The authors conclude that dose adjustments are probably not required when injectable GLP-1 medicines are used with oral medicines, while adding that the results should be applied with care in people with kidney problems or those taking narrow-therapeutic-index drugs.

Side effects and people who stopped

This review looked at drug levels, not side effects. The summary we reviewed does not report side effects or the number of people who stopped any medicine.

What this study does not tell you

• Most studies enrolled healthy volunteers. People with obesity, diabetes or kidney disease may absorb medicines differently, and the review notes a lack of data in those groups.
• Some evidence came from prescribing sheets rather than full published studies, which limits how closely it could be checked.
• The review did not pool results, so there is no single estimate of how much peak levels changed.
• The review covered injectable GLP-1 receptor agonists. Its findings should not be applied to tirzepatide, which was not included in the extracted data.
• The review cannot tell any individual how to time or dose their own medicines. That is a question for a clinician or pharmacist who knows the full medicine list.
• The summary we reviewed does not state who funded the review.

What it means in Canada

For Canadian readers who take a GLP-1 injection alongside other pills, this review is broadly reassuring about total drug exposure, but it comes with important limits. It is based mostly on healthy volunteers and does not cover every medicine. Questions about the contraceptive pill, blood thinners or heart medicines are ones a pharmacist can answer for your own medicine list. For how these medicines are used in Canada, see semaglutide in Canada and tirzepatide in Canada. For coverage questions, see public drug coverage by province.

Related reading

Other studies in this library that bear on the same question.

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