METACYCLINE 1% EYE OINTMENT

Dosage

Children: For paediatric dosing, refer to the BNF for Children.

Adults: For specific dosing information, please refer to the official prescribing information or BNF.

Mechanism of action

The specific mechanism of action for hydrochloride salts depends on the active ingredient they are associated with. Generally, hydrochloride salts improve drug solubility and bioavailability, facilitating absorption in the gastrointestinal tract. For example, in the case of antihistamines, the mechanism involves blocking H1 receptors, while in antidepressants, it may involve the inhibition of neurotransmitter reuptake.

Pharmacodynamics

Pharmacodynamics varies significantly depending on the active drug component. Hydrochloride salts may exhibit effects such as sedation, analgesia, or modulation of neurotransmitter levels, depending on their therapeutic class. The onset and duration of action will also depend on the specific pharmacological properties of the drug.

Pharmacokinetics

Pharmacokinetics is influenced by the nature of the active drug and its hydrochloride form. Generally, hydrochloride drugs are absorbed in the gastrointestinal tract, metabolized primarily in the liver, and excreted via the kidneys. The specific absorption rates, half-lives, and clearance rates will depend on the characteristics of the active compound.

Contra-indications

• Hypersensitivity to hydrochloride or any component of the formulation
• Severe renal impairment
• Severe hepatic impairment

Adverse effects

• Nausea
• Vomiting
• Diarrhea
• Constipation
• Dizziness
• Headache
• Fatigue
• Hypotension
• Electrolyte imbalances

Interactions

• May interact with other antihypertensive agents leading to additive effects
• Potential interaction with diuretics, increasing the risk of electrolyte disturbances
• May enhance the effects of central nervous system depressants

Precautions

• Use with caution in patients with a history of renal or hepatic impairment
• Monitor renal function and electrolytes regularly
• Assess potential risks in patients with a history of cardiovascular disease
• Consider potential drug interactions with concurrent medications

Pregnancy

Use during pregnancy only if the potential benefit justifies the potential risk to the fetus. Consult relevant guidelines for specifics.

Breast-feeding

Use with caution, as it may pass into breast milk. Monitor the infant for any adverse effects.

Storage

Store at room temperature, away from moisture and heat. Keep out of reach of children.

Formulations

• Tablets
• Oral solution
• Injection

Indications

• Bacterial infections such as chlamydia
• Rickettsial infections
• Mycoplasma infections
• Acne vulgaris (off-label use)

Dosage

Children: In children aged 12 to 17 years, the recommended dose for susceptible infections is 100 mg daily, but this can vary based on specific circumstances and should be guided by the BNF for Children

Adults: For susceptible infections, the typical adult dose is 100 mg twice daily for 5 to 14 days, depending on the specific infection. For acne, 500 mg twice daily for 6 to 12 weeks may be used.

Mechanism of action

Tetracycline works by binding to the 30S ribosomal subunit of bacteria, inhibiting the attachment of aminoacyl-tRNA to the ribosomal acceptor site. This effectively prevents protein synthesis, which is essential for bacterial growth and replication.

Pharmacodynamics

The pharmacodynamics of tetracycline involves its bacteriostatic action, meaning it stops bacteria from multiplying rather than killing them outright. Its spectrum of activity covers a wide range of gram-positive and gram-negative bacteria, as well as some atypical pathogens. Resistance can develop through various mechanisms, including efflux pumps and ribosomal protection.

Pharmacokinetics

Tetracycline is well-absorbed from the gastrointestinal tract, although its absorption can be decreased by the presence of divalent and trivalent metal ions (such as calcium, magnesium, and iron). It is distributed widely in body tissues and fluids, with a volume of distribution of approximately 0.5-1.0 L/kg. Tetracycline is largely excreted unchanged in the urine, with a half-life ranging from 6 to 12 hours. Renal impairment can increase its half-life, necessitating caution in dosing.

Contra-indications

• Children under 12 years due to deposition in growing bone and teeth, causing staining and occasionally dental hypoplasia

Adverse effects

• Gastrointestinal disturbances
• Hearing impairment
• Discoloration of teeth
• Photosensitivity
• Dizziness
• Vertigo
• Skin reactions such as rash or pruritus
• Allergic reactions including anaphylaxis
• Esophagitis
• Stomatitis
• Pseudomembranous colitis

Interactions

• Strontium and tetracyclines: Severe (decreases absorption)
• Fosphenytoin and tetracyclines: Moderate (decreases concentration)
• Tetracyclines and lithium: Moderate (increases risk of lithium toxicity)
• Rifampicin and tetracyclines: Moderate (decreases exposure)
• Mitotane and tetracyclines: Unknown (decreases exposure)
• St John's Wort and tetracyclines: Unknown (decreases exposure)
• Tetracycline and antimalarials: Unknown (decreases concentration)
• Tetracycline and atovaquone: Unknown (decreases concentration)
• Tetracycline and relugolix: Unknown (increases exposure)

Precautions

• Use with caution in patients with renal impairment, as dose adjustments may be necessary
• Monitor for hepatotoxicity, pigmentation, and systemic lupus erythematosus in long-term use
• Avoid concurrent use with medications that may affect absorption or increase toxicity
• Advise patients to remain upright for at least 30 minutes after taking to prevent esophageal irritation

Pregnancy

Tetracyclines are generally not recommended during pregnancy due to the risk of dental staining and skeletal abnormalities in the developing fetus.

Breast-feeding

Tetracyclines can be excreted in breast milk and may affect the infant's dental development; caution is advised.

Storage

Store in a cool, dry place, away from direct sunlight and moisture. Keep out of reach of children.

Formulations

• Tablets: 250 mg, 500 mg
• Oral suspension