There is a very bold distinction in terminology to observe between impotence – or erectile dysfunction –, and fertility or changes to the testis and spermatozoa.
The study in question does not say anything about impotence but reported a
The results clearly demonstrate that aqueous extracts from dried calyx of H. sabdariffa, either cold or boiled, alter normal sperm morphology and testicular ultrastructure and adversely influence the male reproductive fertility in albino mice.
This cautionary tale is an interesting result and has to be followed up, just like the authors conclude, not in the least, because:
The results also suggest that the apparent safety of this herb should be re-evaluated and precautions taken against heavy over-consumption as a beverage, although the sensitivity toward this herb may be different for man in relation to rodents and care must be taken in extrapolating animal experimental studies with humans.
These results are not only new but solitary. Not only is this herb apparently used abundantly and already for a very long time, the traditional folk wisdom for this herb has no adverse effects recorded. While that may not count as much, it is a weak indicator not to be dismissed entirely.
Contrasting to Mahmoud's findings, a recent review found
Additionally, deleterious effects on the testis and spermatozoa and an adverse influence in the male reproductive fertility of albino mice were also reported after a cHs WE was administered daily for 4 weeks in a dose of 200 mg/kg (Y. I. Mahmoud, 2012). In contrast to these studies, long term administration of Hs WE for 10 weeks and hibiscus anthocyanins (50–200 mg/kg b.w.) for 5 days did not affect the male reproductive system in rats (Ali et al., 2012).[…]
Based on the data presented above, dosages up to 200 mg/kg should be safe and not show signs of toxicity, but further studies, most importantly with chemically well characterised extracts, are warranted.[…]
Taken together, the data available today from preclinical and clinical studies does not provide substantiated evidence of any therapeutically relevant interaction potential of commonplace teas or beverages containing hibiscus and its preparations. This complements the evidence based on the complete absence of drug interaction case reports involving hibiscus in the scientific literature.
From: Inês Da-Costa-Rocha et al.: "Hibiscus sabdariffa L. – A phytochemical and pharmacological review", Food Chemistry 165 (2014) 424–443, DOI: https://doi.org/10.1016/j.foodchem.2014.05.002
Most of the available data indicates a large range of overall safety for this herb. Even the systematic review above bases its evaluation on the Mahmoud paper in concluding that 200mg/kg body weight per day should be of no concern.
But the Mahmoud paper has several grave weaknesses:
- the whole shebang of scientific procedures is based on a single batch of herbs
- this batch was procured on a local market
- the batch was not tested for pesticides or similar adulterations
- the batch was chemically not very well defined at all, but just used as is, phytochemicals in natural products vary in sometimes astounding amounts
- no hint of a possible mechanism of action was given
Other studies on Hibiscus species come to almost diametrically opposed conclusions regarding infertility for sabdariffa:
Idris: Protective role of Hibiscus sabdariffa calyx extract against streptozotocin induced sperm damage in diabetic rats. 2012
Or regarding impotence, as one herb traditionally used against impotence in male was reported as Hibiscus mutabilis (Ethnomedicobotanical study of indigenous knowledge on medicinal plants used for the treatment of reproductive problems in Nalbari district, Assam, India, 2018)
There is no evidence presented for this hibiscus species (sabdariffa) as a cause for impotence. A single study came to the conclusion that Hibiscus sabdariffa extract might interfere with rodent fertility.
Even if rodents displayed detrimental effects, which are nowhere sure to be translatable into human effects, in this unreplicated, not followed-up, single study with its severe limitations, even rodents might rest assured until:
- the effects are replicated
- in human cells at least
- the chemical makeup of samples is ascertained
- possible adulterations are ruled out
- (a mechanism of action is proposed)
Taking too large doses of anything is not recommended anyway. For this herb it seems as if two cups a day will not do much harm.