GLP-1 and Your Other Medications: Interactions Every Patient Should Know

GLP-1 medications interact with other drugs primarily through one mechanism: delayed gastric emptying. When food and oral medications sit in your stomach longer, absorption timing changes. For most medications, this isn’t clinically significant. For a few, it matters a lot.

Blood Pressure Medications

GLP-1 medications lower blood pressure independently — semaglutide reduces systolic BP by 3–5 mmHg on average. If you’re already on antihypertensives, this additive effect may cause your blood pressure to drop too low (hypotension). Symptoms include dizziness, lightheadedness, and fatigue. Your provider may need to reduce your BP medication doses as you lose weight. Monitor your blood pressure at home, especially during the first months of GLP-1 treatment.

Thyroid Medications (Levothyroxine)

This is one of the most clinically important interactions. Oral semaglutide (Rybelsus) increases levothyroxine exposure by approximately 33% — meaning your thyroid hormone levels may rise above target if you don’t adjust timing. Injectable semaglutide and tirzepatide have less impact on levothyroxine absorption, but delayed gastric emptying can still alter absorption timing. Take levothyroxine on an empty stomach, at least 30–60 minutes before any food, and ideally at a consistent time relative to your GLP-1 dose. Get your thyroid levels checked 6–8 weeks after starting GLP-1 treatment.

Oral Contraceptives

Tirzepatide reduces oral contraceptive absorption by approximately 20%. This means oral birth control pills may be less effective while on tirzepatide. The FDA recommends switching to a non-oral contraceptive method (IUD, implant, patch, ring, injection) or adding a barrier method for 4 weeks after starting tirzepatide and after each dose increase. Semaglutide does not appear to have this interaction.

Insulin and Diabetes Medications

If you’re on insulin or sulfonylureas (glipizide, glyburide), adding a GLP-1 medication increases hypoglycemia risk. Your provider should reduce insulin doses by 20–30% when starting GLP-1 treatment and adjust further based on blood glucose monitoring. Metformin does not carry hypoglycemia risk and can generally be continued without adjustment.

Blood Thinners (Warfarin)

Delayed gastric emptying can alter warfarin absorption, potentially affecting INR levels. If you take warfarin, your provider should check INR more frequently during the first months of GLP-1 treatment and after dose changes.

The General Rule

Any oral medication that has a narrow therapeutic window — where small changes in absorption significantly affect efficacy or safety — deserves attention when you start a GLP-1 medication. Bring your complete medication list to your prescriber and discuss each one.