CBD and Antiviral Medications (e.g. Tamiflu, Zovirax)
David ReichUpdated:Key points at a glance:
CBD may interact with antiviral medicines such as Tamiflu and Zovirax, although the overall risk is generally considered low. CBD can influence certain enzymes involved in drug metabolism, including CYP2C19 and CYP3A4, and taking products at different times (for example, two to three hours apart) does not reliably rule out possible interactions. Oseltamivir is mainly activated via the enzyme CES1, while aciclovir is largely excreted unchanged by the kidneys. If using CBD, many people start with a low daily amount, such as 5 to 10 mg, and increase gradually if needed. It is important to monitor for possible side effects and discuss any use of CBD alongside prescribed medication with a doctor.
In recent years, cannabidiol (CBD) has attracted considerable attention in relation to its potential effects on wellbeing. CBD oils are particularly popular. It is important, however, to understand possible interactions between CBD and other medicines, including antiviral medicines such as Tamiflu (oseltamivir) and Zovirax (aciclovir). You should always consult your doctor before adding CBD to your current medication.
How does CBD work in the body?
CBD modulates the endocannabinoid system (ECS) without binding directly to the CB1 receptor. It also influences receptors such as TRPV1 and 5‑HT1A as well as the adenosine system, and can inhibit enzymes/transporters involved in drug metabolism (in particular CYP2C19 and CYP3A4; there are also indications of effects on UGT1A9/UGT2B7). In practical terms, CBD may increase the plasma levels of some medicines or slow their breakdown. These effects are dose‑dependent and vary according to the individual situation and concomitant medication. Other factors such as liver function, age and polypharmacy also play a role.
CBD has a regulatory effect on the ECS and may inhibit drug‑metabolising enzymes such as CYP2C19 and CYP3A4. This means CBD drug interactions are possible – especially at higher doses and with multiple prescription medicines.
How do antiviral medicines like Tamiflu and Zovirax work?
Tamiflu (oseltamivir)
Oseltamivir is a prodrug that is converted in the liver primarily by carboxylesterase 1 (CES1) into its active form (oseltamivir carboxylate) – not by CYP‑450 enzymes. It is used for the treatment and prevention of influenza. The active form is mainly excreted via the kidneys. A CYP‑mediated interaction with CBD is therefore considered less likely.
Zovirax (aciclovir)
Aciclovir inhibits viral DNA polymerase and is largely excreted unchanged via the kidneys. CYP‑dependent interactions are unlikely; kidney function and adequate hydration are more relevant – particularly at higher doses or with intravenous administration.
Tamiflu is activated via CES1 (not a typical CYP pathway), while Zovirax (aciclovir) is mainly excreted via the kidneys. Metabolic interactions with CBD CYP3A4 or CBD CYP2C19 pathways therefore appear rather unlikely.
Possible interactions: CBD + antiviral medicines
Mechanisms of CBD and antiviral drug interaction
CBD can inhibit CYP2C19 and CYP3A4, and there are indications of effects on UGT enzymes (UGT1A9/UGT2B7). Theoretically, this may slow the breakdown of some medicines and increase their effects/side effects. Taking products a few hours apart (2–3 hours) does not reliably prevent such metabolic interactions, but may spread out additive side effects (e.g. nausea, drowsiness). Data on direct clinical interactions with oseltamivir or aciclovir are lacking; mechanistic considerations suggest a low risk – individual factors remain crucial. 3
There are no direct studies on CBD with Tamiflu or CBD with Zovirax. Mechanistically, the interaction risk appears low, but you should still pay attention to symptoms and discuss CBD use with a doctor, especially if you take other prescription medicines.
CBD and Tamiflu (oseltamivir): what does the evidence say?
Oseltamivir is mainly converted to its active form via CES1, not via CYP enzymes. A CYP‑mediated interaction with CBD is therefore considered less likely. As CBD may still influence other enzymes/transporters and contribute to side effects (e.g. nausea, tiredness), medical advice is sensible. Start with a low dose of CBD and monitor tolerability during the first few days.
CBD and Zovirax (aciclovir): what does the evidence say?
Aciclovir is predominantly eliminated unchanged by the kidneys. A metabolic interaction with CBD is regarded as unlikely. Ensure adequate fluid intake and report any new symptoms (e.g. dizziness, nausea). Particular caution is recommended in people with pre‑existing kidney impairment.
| Active substance | Metabolism/elimination | Potential overlap with CBD | Practical assessment |
|---|---|---|---|
| Oseltamivir (Tamiflu) | Prodrug; activation via CES1; active form renally eliminated; low CYP involvement | CBD inhibits CYP2C19/CYP3A4 (probably less relevant here); possible additive side effects (tiredness, nausea) | Low CBD interaction risk; seek medical advice, start with a low CBD dose, monitor symptoms |
| Aciclovir (Zovirax) | Predominantly excreted unchanged via the kidneys | No relevant CYP overlap; caution in renal impairment; hydration important | Very low interaction risk with CBD; ensure sufficient fluid intake and watch for new symptoms |
Further background on CBD interactions with medicines.
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Practical guide for patients using CBD with antiviral medicines
Dosage, timing and monitoring
- Start low, go slow: begin with 5–10 mg CBD daily (e.g. oil or capsules) and only increase if well tolerated. Dosage tips: how to dose CBD.
- Leaving a 2–3 hour gap between CBD and antiviral medicines can reduce overlapping side effects, but does not reliably prevent CYP/UGT interactions.
- During acute infections, try to make as few changes as possible to your medication; if starting CBD, consider beginning with a low dose.
- During the first 3–5 days, watch for: tiredness/drowsiness, dizziness, nausea, diarrhoea, skin reactions.
- If you have a history of liver problems or are taking other potentially hepatotoxic medicines: consider having liver function tests checked after 2–4 weeks.
- Warning signs – pause intake and consult a doctor: persistent diarrhoea, severe rash, newly developed severe tiredness, yellowing of skin/eyes, dark urine.
Who should be particularly cautious with CBD and prescription medicines?
- Pregnant or breastfeeding people
- People with liver or kidney disease
- Older individuals and people taking multiple medicines
Product quality
- Check proof of quality (COA/certificate of analysis): content, purity, residues.
- Pay attention to THC content (ideally none or <0.2%).
- Full spectrum vs isolate: start with a low dose and observe your individual response.
Tip: Check the COA: no or <0.2% THC, tested purity.
Side effects of CBD
The most commonly reported side effects include tiredness, drowsiness, dry mouth, dizziness and gastrointestinal complaints such as nausea or diarrhoea. In clinical studies with high‑dose CBD (Epidiolex), diarrhoea, tiredness and reduced appetite occurred in more than 10% of participants; temporary increases in liver enzymes have been observed – particularly in combination with valproic acid. Over‑the‑counter CBD products are usually taken at lower doses, but it is still advisable to start with small amounts. Further details: CBD side effects.
CBD is generally considered to be well tolerated. Tiredness and gastrointestinal complaints are relatively common; liver enzymes may occasionally rise – particularly at higher doses, or in certain combinations with other medicines that affect liver enzymes.
Side effects of antiviral medicines
Common side effects include nausea, vomiting, diarrhoea, headache and tiredness. Oseltamivir has occasionally been associated with neuropsychiatric symptoms; aciclovir can, in rare cases and particularly with inadequate hydration, contribute to kidney problems related to crystalluria. If you experience new or persistent complaints, seek medical advice. For details, refer to the official product information (SmPC) for oseltamivir and aciclovir.
Conclusion
For the combination of CBD with Tamiflu (oseltamivir) or Zovirax (aciclovir), mechanistic considerations point to a low to very low interaction risk. Nevertheless, dosage, individual factors and potential additive side effects should be taken into account. Start with a low dose, monitor your symptoms and consult your doctor if you are unsure.
Disclaimer: This article is for information purposes only and does not replace medical advice. Do not change medicines or dosages without consulting your doctor. No claims are made regarding the treatment or cure of illnesses.