Herbal tea with blood-pressure and diabetes drugs: what do the in-vitro interaction results mean?

Study card

• Country: South Africa.
• City: Bloemfontein.
• Research site: African Medicines Innovations and Technologies Development (AMITD), Department of Pharmacology, Faculty of Health Sciences, University of the Free State.
• Design: in-vitro laboratory study, not a patient trial.
• Published: 22 October 2026.
• Journal: Clinical Phytoscience.
• DOI: 10.1186/s40816-026-00446-0. [1]

What did the researchers ask?

The team studied two commercial Sela products marketed for blood pressure or blood sugar and asked whether their extracts inhibit enzymes connected with those pathways, and what happens when they are combined with established medicines: lisinopril, aliskiren and acarbose. [1]

This matters because people taking medicines for hypertension or diabetes may also use herbal products. However, an enzyme experiment can identify a laboratory interaction signal; it cannot by itself show whether the combination is helpful, harmful or safe in a person.

What products were tested?

Sela High Blood (SHB) contained black tea Camellia sinensis, rooibos Aspalathus linearis and hibiscus. Sela Blood Sugar (SBS) contained rooibos, black tea and Aloe ferox. The commercial teabags were donated to AMITD at the University of the Free State. [1]

For extraction, six 1.7-g teabags (10.2 g total) were infused in 1,500 mL boiling water for 15 minutes, filtered, frozen and freeze-dried into a powder. The extraction was performed in triplicate. [1]

How was preliminary safety assessed?

HEK-293 cells were used for SHB and Chang liver cells for SBS in an MTT cell-viability assay. Viability remained above 75% across the tested ranges. That is a preliminary cell result, not proof that drinking the products is safe for patients, safe long term, or safe with medicines. [1]

Which enzymes and drugs were tested?

SHB was tested against:

• angiotensin-converting enzyme (ACE) with lisinopril;
• renin with aliskiren.

SBS was tested against:

• α-amylase;
• α-glucosidase; with acarbose.

Each condition was assessed in three independent biological experiments; each concentration also had three technical wells. Therefore the statistical experimental n was 3 independent experiments per condition, not a number of patients. [1]

Main results

SHB inhibited ACE with IC50 = 247.60 µg/mL and renin with IC50 = 28.38 µg/mL.

SBS inhibited α-amylase with IC50 = 1974.00 µg/mL and α-glucosidase with IC50 = 4.59 µg/mL. [1]

Some IC50 values fell when tea extract and drug were combined, meaning stronger enzyme inhibition under the experimental conditions. Reported differences included p=0.0076 for SHB plus lisinopril in the ACE assay and p=0.0037 for SHB plus aliskiren in the renin assay; significant changes were also reported for SBS with acarbose in the carbohydrate-digesting enzyme assays. [1]

Does that mean the tea strengthens the medicine in patients?

No clinical conclusion can be drawn from this study.

A lower IC50 in an enzyme system means stronger inhibition under those laboratory conditions. It does not tell us whether tea constituents reach comparable concentrations in the human body, how digestion and metabolism alter them, whether blood pressure or blood glucose changes, whether drug concentrations change, or whether combining the products increases adverse effects such as hypotension or hypoglycaemia. No pharmacokinetic or human drug-interaction study was performed. [1]

Strength of evidence

The paper is peer reviewed and useful for generating a drug-interaction hypothesis, but its clinical evidence strength is low because it is in vitro. The result is enzyme inhibition, not a health outcome.

The findings are also specific to two commercial multi-herb products; they cannot be generalized to black tea, rooibos, hibiscus or aloe individually, or to herbal teas as a class.

It is said… what does the evidence show?

Claim: If an herbal tea makes a medicine inhibit an enzyme more strongly in the lab, combining them is therapeutically better.

Verdict: Not established.

Explanation: A test-tube interaction is a reason to investigate human exposure, safety and outcomes; it is not evidence of clinical benefit. [1]

Practical meaning

The study does not justify adding Sela tea to lisinopril, aliskiren or acarbose to “boost” treatment, and it does not justify changing a medicine dose. People using multi-herb products with hypertension or diabetes medicines should make the product composition known to their clinician or pharmacist because clinical interaction data remain insufficient.

Funding and conflicts

The research was funded by the AMITD Platform. The authors acknowledged South Africa’s Department of Science, Technology and Innovation, the University of the Free State and the Technology Innovation Agency. They declared no competing interests. The commercial Sela products were donated by the manufacturer, a relevant contextual fact even though no competing interest was declared. [1]

Conclusion

The two Sela tea extracts inhibited selected enzymes in vitro, and some enzyme inhibition increased when extracts were combined with lisinopril, aliskiren or acarbose. The study does not establish a clinical drug interaction, additional therapeutic benefit or safety of co-use in patients. Human pharmacokinetic, efficacy and safety studies would be needed.