Drug interaction
Phenobarbital and Doxycycline Interaction in Dogs: Risks & Management
In dogs, phenobarbital may enhance the metabolism of doxycycline and therefore decrease the effect of doxycycline.[3][4][5]
Phenobarbital has been shown to shorten the half-life of doxycycline for as long as 2 weeks after barbiturate therapy is discontinued.[1][2]
If phenobarbital and doxycycline are administered concurrently, the clinical response to doxycycline should be monitored closely.[1][2]
The interaction
The key interaction is that phenobarbital may reduce doxycycline exposure and effect.[1][2][3][4][5]
- Effect in dogs: In dogs, phenobarbital may enhance the metabolism of doxycycline and therefore decrease the effect of doxycycline.[3][4][5]
- Half-life: Phenobarbital has been shown to shorten the half-life of doxycycline for as long as 2 weeks after barbiturate therapy is discontinued.[1][2]
Interaction table
The reported effect is reduced doxycycline exposure or effect with concurrent phenobarbital.[1][2][3][4][5]
| Drugs | Severity (as the source states it) | Effect | Management |
|---|---|---|---|
| Phenobarbital + doxycycline | — | In dogs, phenobarbital may enhance the metabolism of doxycycline and therefore decrease the effect of doxycycline.[3][4][5] Phenobarbital has been shown to shorten the half-life of doxycycline for as long as 2 weeks after barbiturate therapy is discontinued.[1][2] | If phenobarbital and doxycycline are administered concurrently, the clinical response to doxycycline should be monitored closely.[1][2] |
Mechanism
The stated mechanism is hepatic enzyme induction by phenobarbital.[1][2]
- Enzyme induction: This mechanism is probably through the induction of hepatic microsomal enzymes that metabolize the antibiotic.[1][2]
- Metabolism: Phenobarbital may enhance the metabolism of doxycycline.[3][4][5]
Monitoring & management
The main management point is to monitor doxycycline response closely during concurrent use.[1][2]
- Concurrent use: If phenobarbital and doxycycline are administered concurrently, the clinical response to doxycycline should be monitored closely.[1][2]
- After discontinuation: Phenobarbital has been shown to shorten the half-life of doxycycline for as long as 2 weeks after barbiturate therapy is discontinued.[1][2]
Species notes
A dog-specific product information source describes reduced doxycycline effect with phenobarbital.[3]
- Dogs: Phenoleptil Tablets for Dogs states that phenobarbital may enhance the metabolism of doxycycline and that the effect of doxycycline may therefore be decreased.[3]
Frequently asked questions
Does phenobarbital reduce the effect of doxycycline in dogs?
What mechanism is reported for the phenobarbital-doxycycline interaction?
How long can phenobarbital affect doxycycline after phenobarbital is stopped?
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Dr. Ravi Baraiya, B.V.Sc. & A.H.
Veterinarian · Reviewed 26 Oct 2026
References
- Label: PHENOBARBITAL elixir - DailyMed
Passage read from this source
- Corticosteroids–Barbiturates appear to enhance the metabolism of exogenous corticosteroids, probably through the induction of hepatic microsomal enzymes. Patients stabilized on corticosteroid therapy may require dosage adjustments if barbiturates are added to or withdrawn from their dosage regimen. - Griseofulvin–Phenobarbital appears to interfere with the absorption of orally administered griseofulvin, thus decreasing its blood level. The effect of the resultant decreased blood levels of griseofulvin on therapeutic response has not been established. However, it would be preferable to avoid concomitant administration of these drugs.
- Doxycycline–Phenobarbital has been shown to shorten the half-life of doxycycline for as long as 2 weeks after barbiturate therapy is discontinued. This mechanism is probably through the induction of hepatic microsomal enzymes that metabolize the antibiotic. If phenobarbital and doxycycline are administered concurrently, the clinical response to doxycycline should be monitored closely.
- Phenytoin, Sodium Valproate, Valproic Acid–The effect of barbiturates on the metabolism of phenytoin appears to be variable. Some investigators report an accelerating effect, whereas others report no effect. Because the effect of barbiturates on the metabolism of phenytoin is not predictable, phenytoin and barbiturate blood levels should be monitored more frequently if these drugs are given concurrently. Sodium valproate and valproic acid increase the phenobarbital serum levels; therefore, phenobarbital blood levels should be closely monitored and appropriate dosage adjustments made as clinically indicated. - CNS Depressants–The concomitant use of other CNS depressants, including other sedatives or hypnotics, antihistamines, tranquilizers, or alcohol, may produce additive depressant effects.
- Monoamine Oxidase Inhibitors (MAOIs)–MAOIs prolong the effects of barbiturates, probably because metabolism of the barbiturate is inhibited. - Estradiol, Estrone, Progesterone, and other Steroidal Hormones–Pretreatment with or concurrent administration of phenobarbital may decrease the effect of estradiol by increasing its metabolism. There have been reports of patients treated with antiepileptic drugs (e.g., phenobarbital) who become pregnant while taking oral contraceptives. An alternate contraceptive method might be suggested to women taking phenobarbital.
- Phenobarbital Sodium Injection, USP CIV FOR IM OR ...
Passage read from this source
Phenobarbital has been shown to shorten the half-life of doxycycline for as long as 2 weeks after barbiturate therapy is discontinued. This mechanism is probably through the induction of hepatic microsomal enzymes that metabolize the antibiotic. If phenobarbital and doxycycline are administered concurrently, the clinical response to doxycycline should be monitored closely.
- Clinical particulars - Phenoleptil® Tablets for Dogs
Passage read from this source
The plasmatic concentration of cyclosporine, thyroid hormones and theophylline is decreased in the case of concurrent administration of phenobarbital. The effectiveness of these substances is diminished too. Cimetidine and ketoconazole are inhibitors of hepatic enzymes: concurrent use with phenobarbital can induce an increase of serum concentration of phenobarbital. Concurrent use with potassium bromide increases the risk of pancreatitis. Concurrent use with other drugs having a central depressive action like narcotic analgesics, morphinic derivates, phenothiazines, antihistamines, clomipramine and chloramphenicol can increase the effect of phenobarbital. Phenobarbital may enhance the metabolism of, and therefore decrease the effect of, antiepileptics, chloramphenicol, corticosteroids, doxycycline, beta blockers and metronidazole. The reliability of oral contraceptives is lower.
- Contra-indications, warnings, etc - Soliphen 60mg tablets ...
Passage read from this source
Cimetidine and ketoconazole are inhibitors of hepatic enzymes: concurrent use with phenobarbital can induce an increase of serum concentration of phenobarbital. Concurrent use with potassium bromide increases the risk of pancreatitis. Concurrent use with other drugs having a central depressive action (like narcotic analgesics, morphinic derivates, phenothiazines, antihistamines, clomipramine and chloramphenicol) can increase the effect of phenobarbital. Phenobarbital may enhance the metabolism of, and therefore decrease the effect of, antiepileptics, chloramphenicol, corticosteroids, doxycycline, beta blockers and metronidazole. The reliability of oral contraceptives is lower. Phenobarbital may decrease the absorption of griseofulvin.
- Revised: July 2026 AN: 04225/2025 Page 1 of 9
Passage read from this source
A therapeutic dose of phenobarbital for antiepileptic therapy can significantly induce plasma protein (such as α1acid glycoprotein, AGP), which bind drugs. Therefore, special attention must be paid to the pharmacokinetics and doses of drugs simultaneously administered. The plasmatic concentration of cyclosporine, thyroid hormones and theophylline is decreased in the case of concurrent administration of phenobarbital. The effectiveness of these substances is diminished, too. Concurrent use with potassium bromide increases the risk of pancreatitis. Concurrent use with other drugs having a central depressive effect can result in an increase of the effect of central depressive drugs. Phenobarbital may enhance the metabolism of, and therefore decrease the effect of, antiepileptics, chloramphenicol, corticosteroids, doxycycline, beta blockers and metronidazole.
- Survival Guide: Common Drug Interactions in Small Animal Medicine
Passage read from this source
Cyclosporine: Potential to interact with many drugs; concurrent use of St. John’s Wort in dogs should be avoided. Phenobarbital: Speeds metabolism of many drugs; increases levetiracetam clearance in dogs.
For veterinary professionals. Information supports — never replaces — clinical judgement; always check the current product label and local regulations.