Hops

Hops (Humulus lupulus) increases GABAergic neurotransmission via bitter alpha-acids (humulones) and their metabolite 2-methyl-3-buten-2-ol. This central inhibitory mechanism is pharmacodynamically synergistic with benzodiazepines, barbiturates, opioids, and antihistamines. Medscape documents multiple pharmacodynamic synergism alerts with hops and CNS depressants including pentobarbital, phenobarbital, and antihistamines.

Avoid combining hops supplements with CNS depressants unless under medical supervision. Warn patients about additive sedation and impaired psychomotor function. This is particularly relevant for patients on benzodiazepines for anxiety or sleep. Do not combine hops with opioids or barbiturates. Caution patients against driving or operating heavy machinery.

In vitro studies show that 8-prenylnaringenin from hops potently inhibits CYP2C9 with an IC50 of 1.1 μM, and the whole hop extract inhibits CYP2C9 by 88% at 5 μg/mL (IC50 = 0.9 μg/mL; Yuan et al., Eur J Pharm Sci 2014). Since S-warfarin is primarily cleared by CYP2C9, hop supplementation may increase warfarin plasma levels and INR, increasing bleeding risk.

Monitor INR when patients on warfarin initiate hops supplementation. Review all CYP2C9-dependent medications in patients starting hops extract. Advise patients on warfarin that hops supplements may require warfarin dose reduction. The effect is dose-dependent; beer consumption is unlikely to be clinically significant, but medicinal hop extracts pose greater risk.

8-Prenylnaringenin (8-PN) from hops is one of the most potent phytoestrogens known, more estrogenic than genistein or daidzein. It binds estrogen receptors (ERα and ERβ) and may compete with or additively potentiate the estrogenic effects of combined oral contraceptives or HRT, potentially disrupting hormonal balance and altering menstrual cycle regulation.

Advise women on oral contraceptives or HRT to use hops supplements cautiously. Theoretically, hops phytoestrogens may augment estrogenic effects (breast tenderness, bloating, mood changes) or, through receptor competition, reduce contraceptive efficacy. Patients with hormone-sensitive conditions (breast cancer history, endometriosis) should avoid hops supplements.

8-Prenylnaringenin from hops produces time-dependent (mechanism-based) inactivation of CYP1A2, which is the principal enzyme metabolising theophylline, clozapine, and olanzapine. Unlike competitive inhibition, mechanism-based inactivation is irreversible until new enzyme is synthesised (~2-3 days), meaning the interaction may persist after hops discontinuation.

Monitor theophylline plasma levels in patients on this narrow-therapeutic-window bronchodilator who use hops supplements. For patients on clozapine or olanzapine, monitor for signs of antipsychotic toxicity (excessive sedation, hypotension, metabolic changes). Dose adjustment of CYP1A2-dependent medications may be required. Allow at least 3 days after hops discontinuation before assuming CYP1A2 function has recovered.

Hops extract at 5 μg/mL inhibits CYP2C8 by 93% (IC50=0.8 μg/mL) and CYP2C19 by 70% (IC50=3.3 μg/mL) in vitro (Yuan et al. 2014). CYP2C8 is critical for paclitaxel and repaglinide clearance; CYP2C19 activates clopidogrel prodrug to its active metabolite. Inhibition of CYP2C19 by hops may reduce clopidogrel antiplatelet efficacy, increasing thrombotic risk.

Patients on paclitaxel chemotherapy should avoid hops supplements due to risk of drug accumulation and toxicity. Patients on clopidogrel (which requires CYP2C19 activation) should be informed that hops may reduce antiplatelet efficacy. Monitor platelet function if clinically relevant. For repaglinide, monitor blood glucose. Seek oncology or cardiology guidance before permitting co-use.

8-Prenylnaringenin, the potent phytoestrogen in hops, may competitively bind estrogen receptors and partially antagonise the anti-estrogenic action of tamoxifen in oestrogen receptor-positive (ER+) breast cancer. Tamoxifen works by blocking ER; hops phytoestrogens may provide residual ER stimulation even in the presence of tamoxifen, potentially compromising cancer therapy.

Women receiving tamoxifen or aromatase inhibitors for ER+ breast cancer should avoid hops supplements entirely. The phytoestrogen content (8-PN) could undermine cancer treatment efficacy. This recommendation applies to other phytoestrogen-containing herbal supplements. Counsel patients clearly and document advice in medical records.