Clinical Pharmacy Cases - Drug Interactions and Dosing Considerations

Vocabulary terms covering high-yield clinical pharmacy cases, drug-drug interactions, dosing adjustments, contraindications, and management strategies.

Apixaban AF Dose Reduction Criteria

A reduction to $2.5\,\text{mg}\text{ bd}$ in atrial fibrillation indicated when at least 2 of the following factors are present: weight $<60\,\text{kg}$, age $>80\text{ years}$, or serum creatinine $>133\,\mu\text{mol/L}$.

Apixaban and Enzyme Inducers Interaction

Combination with strong inducers (e.g., St John's wort, rifampicin, carbamazepine, phenytoin) is not recommended by the manufacturer and should be used with caution because it halves apixaban concentration and may reduce its efficacy.

Apixaban and Ritonavir Interaction

A combination contraindicated by the manufacturer because ritonavir is a strong CYP3A4 inhibitor that increases apixaban concentration and elevates the risk of bleeding.

Dapagliflozin (FORXIGA) Renal Threshold

Contraindicated if the creatinine clearance ($\text{CrCl}$) is $<60\,\text{mL/min}$.

Dapagliflozin Adverse Effects and Management

Side effects include hypotension, dehydration, and urinary tract infections (UTI); if UTIs are recurrent, the drug should be withheld, with effects typically reversible to normal after the first few weeks.

Phentermine (DUROMINE) Epilepsy Interaction

Phentermine is contraindicated in epilepsy (e.g., in patients taking levetiracetam); the recommended management is switching therapy to orlistat combined with lifestyle modifications.

Phentermine Contraindications

Contraindicated in epilepsy, hyperthyroidism, peptic ulcer, prostatic hypertrophy, hypersensitivity to amines, history of misuse, cardiac disease, within 14 days of a MAOI (e.g., phenelzine, tranylcypromine), and with medications contributing to serotonin toxicity.

Rizatriptan and Propranolol Interaction

Propranolol decreases the metabolism of rizatriptan (likely via MAO inhibition), raising rizatriptan serum levels and risk of toxicity; switching to Zolmitriptan is recommended.

Aripiprazole Injection Initiation (Oral Switch)

When switching from an oral antipsychotic to long-acting aripiprazole injection, continue the oral antipsychotic for 14 days following the first injection.

Aripiprazole Injection Initiation (Long-Acting Switch)

When switching from another long-acting antipsychotic, administer the first aripiprazole injection at the scheduled time of the previous injection together with oral aripiprazole $10\text{--}20\,\text{mg}$ once daily for the first 14 days.

Doxycycline and Oral Isotretinoin Interaction

Concomitant use increases the risk of benign intracranial hypertension; combination must be avoided, and Malarone should be used instead of doxycycline for malaria prophylaxis.

Isotretinoin Precautions and Contraindications

Contraindications and precautions include depression, hypervitaminosis A, hyperlipidemia, diabetes, hepatic impairment, pregnancy, breastfeeding, and laser refractive eye surgery.