The short answer
Licorice containing glycyrrhizin can raise blood pressure on its own. An added blood-pressure effect with oral phenylephrine is plausible, but the exact combination has not been demonstrated in the sources checked. Review oral licorice or Gan cao products before combining them with the medicine, particularly if your blood pressure is high.
What this answer covers
Oral phenylephrine hydrochloride and Radix et Rhizoma Glycyrrhizae, the licorice root/rhizome ingredient. Evidence is relevant chiefly to glycyrrhizin-containing oral preparations; species and processing may vary. DGL and topical licorice are distinct exposures.
- Supplement or preparation
- Radix et Rhizoma Glycyrrhizae
- Medication ingredient
- phenylephrine
Which licorice preparations are relevant?
The main concern is glycyrrhizin, also called glycyrrhizic acid. Oral licorice root products can contain it, while deglycyrrhizinated licorice, or DGL, is processed to remove it. The name Gan cao alone does not establish the dose or content, and topical preparations are a different exposure.
What did the clinical trial show?
Twenty-eight healthy volunteers consumed licorice supplying 100 mg glycyrrhizic acid daily for two weeks in a crossover study. Mean home systolic pressure changed by +3.1 mmHg during licorice use and -0.3 mmHg during control. No phenylephrine was given, so the study demonstrates an independent licorice effect.
Why consider phenylephrine at all?
Some phenylephrine cold medicines are unsuitable for people with hypertension or heart disease. Combining that concern with licorice's independent blood-pressure effect provides a reason for caution, but it remains an inference about this pair. A single-dose phenylephrine study in selected healthy adults cannot resolve the risk with licorice.
Would separating the doses help?
No validated spacing rule was found. Licorice-related effects depend on its content and pattern of use, so a short gap should not be presented as eliminating the concern. Ask a pharmacist to review the exact root product, how long you have used it and any history of hypertension or heart or kidney disease.
Evidence and practical considerations
Side effects & toxicity
An independent blood-pressure effect is supported for a measured glycyrrhizin-containing licorice exposure. Whether phenylephrine adds a clinically important effect with a particular Gan cao preparation remains an untested inference.
- Exact scope
- Oral phenylephrine hydrochloride and Radix et Rhizoma Glycyrrhizae, the licorice root/rhizome ingredient. Evidence is relevant chiefly to glycyrrhizin-containing oral preparations; species and processing may vary. DGL and topical licorice are distinct exposures. with oral phenylephrine hydrochloride supplying the exact phenylephrine active moiety, RxCUI 8163
- Medication form and route
- oral
- Timing or duration
- During co-use; source-specific durations are retained without inferring a universal safe duration.
What remains uncertain
- No direct phenylephrine/licorice clinical study located. The licorice trial was small, abstract-only and preparation-specific; it did not quantify the interaction or establish a universal risk threshold.
Factors that may matter: glycyrrhizin content; duration of licorice use; high blood pressure or heart/kidney disease; species and processing.
Read the supporting source summaries
Source 1: regulatory product information
medicines.org.uk · Source check Sep 10, 2026
Population: People using this oral cold and flu combination product, age 16 years and older.
The product combines phenylephrine and caffeine, warns against excessive additional caffeine, and lists hypertension and heart disease among contraindications. Caffeine and phenylephrine adverse effects include palpitations and sleep-related symptoms.
How this applies: Exact phenylephrine active moiety and caffeine co-formulation. Botanical applicability is an inference only when the supplement actually supplies caffeine; other herb constituents and other phenylephrine routes are not evaluated.
Instructions apply to this finished oral product. They neither establish a universal caffeine threshold or spacing interval nor prove safety of isolated phenylephrine with supplements. Other ingredients confound extrapolation.
Source 2: government botanical evidence summary
nccih.nih.gov · Source check Sep 10, 2026
Population: People using licorice-root products.
Glycyrrhizin can cause serious adverse effects; hypertension, high salt intake and heart or kidney conditions increase concern. DGL is a distinct preparation.
How this applies: Supports constituent- and route-specific risk assessment for Radix et Rhizoma Glycyrrhizae; not proof of phenylephrine-induced amplification.
No exact phenylephrine co-use trial. Broad licorice-root evidence does not specify every Glycyrrhiza species or processed Gan cao preparation.
Source 3: randomized nonblinded crossover trial
pubmed.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: Twenty-eight healthy volunteers, median age 24 years.
Mean home systolic-pressure change was +3.1 mmHg on licorice versus -0.3 mmHg on control; renin and aldosterone were suppressed.
How this applies: Primary human evidence of an independent glycyrrhizin-containing licorice effect; added risk with phenylephrine remains an inference.
No phenylephrine. Full paper was not retrieved; +3.1 mmHg is the within-licorice change, not the between-treatment difference. Does not establish effects for every species, preparation or dose.
Source 4: randomized placebo-controlled crossover study
pmc.ncbi.nlm.nih.gov · Source check Sep 10, 2026
Population: Twenty-eight healthy normotensive adults.
Cardiovascular tolerability was broadly comparable with placebo. Exposure increased more than proportionally with dose; individual concentrations varied substantially.
How this applies: Medication-only evidence prevents presenting every oral phenylephrine dose as inevitably raising blood pressure. It does not show that any supplement combination is safe.
Selected healthy participants, single doses, no supplement co-use. The authors state the design does not adequately assess proarrhythmic risk. Sponsor funded the study and authors were sponsor employees.
Research assessment: · Open full citations ↗
Review and change history
- Publication status
- Published research summary
- Clinical review
- Not yet recorded
| Date | Update |
|---|---|
| Article prepared | |
| Separated licorice's demonstrated independent blood-pressure effect from the unconfirmed combination claim. Added glycyrrhizin/DGL boundaries and corrected interpretation of the crossover study's blood-pressure change. | |
| Research summary published |
Article revision: cebb26c0030e79cb
This guide brings together the evidence records used by our interaction checker. Each finding retains its own preparation limits, research date, and source summaries.
Full citations require verification. Reading this answer and the source summaries does not.
Taking other supplements or medications?
Use the checker to explore your other combinations. Ask a pharmacist to review the exact products, amounts and medical conditions in your complete list.
Open the interaction checkerInformation, not medical advice. Do not change prescribed treatment based on this guide.