Drug Interactions Practice Questions

Q: Renal excretion of lithium is reduced by which of the following medications?

All of the above. **Explanation:** The renal handling of **Lithium** is closely linked to sodium balance because the kidney (specifically the proximal tubule) treats lithium ions similarly to sodium ions. Approximately 80% of filtered lithium is reabsorbed in the proximal tubule. **Mechanism of Interactions:** 1. **Thiazide Diuretics (Hydrochlorothiazide):** These are the most notorious for causing lithium toxicity. By inhibiting sodium reabsorption in the distal tubule, they cause natriuresis (sodium loss). The body compensates by increasing sodium and lithium reabsorption in the **proximal tubule**, leading to a 25–40% increase in serum lithium levels. 2. **NSAIDs (Indomethacin):** Prostaglandins maintain renal blood flow and promote sodium excretion. NSAIDs inhibit prostaglandin synthesis, leading to reduced renal perfusion and increased proximal tubular reabsorption of sodium and lithium. 3. **Loop Diuretics (Furosemide):** While traditionally thought to have a lesser effect than thiazides, loop diuretics can also reduce lithium clearance through compensatory proximal reabsorption due to volume depletion. **Why "All of the above" is correct:** All three classes of drugs decrease the renal clearance of lithium, thereby increasing its plasma concentration and the risk of toxicity. **High-Yield Clinical Pearls for NEET-PG:** * **Safe Antihypertensives:** Calcium Channel Blockers (like Amlodipine) and Centrally acting drugs (like Methyldopa) are generally safer with lithium. * **Drugs to Avoid:** ACE inhibitors and ARBs also significantly increase lithium levels. * **The "Exception" Diuretic:** **Amiloride** is the drug of choice for treating Lithium-induced Nephrogenic Diabetes Insipidus because it blocks the ENaC channels through which lithium enters the collecting duct cells. * **Osmotic Diuretics & Acetazolamide:** These actually *increase* lithium excretion (decreasing levels).

Q: All the following Antiepileptics are inducers except?

Valproate. **Explanation:** The core concept tested here is the effect of antiepileptic drugs (AEDs) on the **Cytochrome P450 (CYP450) enzyme system**. Most traditional AEDs are enzyme inducers, whereas Valproate is the notable exception. **1. Why Valproate is the Correct Answer:** **Valproate** is a potent **enzyme inhibitor**, not an inducer. It inhibits various CYP450 isoenzymes and UDP-glucuronosyltransferase (UGT). Clinically, this is significant because Valproate increases the plasma concentration of other drugs (e.g., it inhibits the metabolism of Phenobarbital and Lamotrigine), potentially leading to toxicity. **2. Analysis of Incorrect Options (Inducers):** * **Phenobarbitone:** A classic, potent inducer of CYP1A2, 2C9, and 3A4. It increases the metabolism of drugs like oral contraceptives and warfarin. * **Phenytoin:** A strong inducer of CYP3A4. It exhibits non-linear (zero-order) kinetics at high doses, but its primary effect on other drugs is induction. * **Rifampicin:** Although primarily an antitubercular drug, it is often included in pharmacology questions as the "prototypical" potent enzyme inducer. **3. High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Enzyme Inducers:** "**G**P **S**teps **P**hone **C**arefully" (**G**riseofulvin, **P**henytoin, **S**moking, **T**erpine, **P**henobarbitone, **C**arbamazepine, **R**ifampicin). * **Mnemonic for Enzyme Inhibitors:** "**VITAMIN K**" (**V**alproate, **I**soniazid, **T**imetidine/Cimetidine, **A**miodarone, **M**acrolides, **I**traconazole, **N**euroleptics, **K**etoconazole). * **Lamotrigine Interaction:** Valproate inhibits the glucuronidation of Lamotrigine, doubling its half-life and increasing the risk of life-threatening **Stevens-Johnson Syndrome (SJS)**.

Q: Inhibitors of serotonin (5-HT) uptake, such as paroxetine, can cause clinically significant drug interactions with which of the following?

Tranylcypromine. **Explanation:** The correct answer is **Tranylcypromine**. **1. Why Tranylcypromine is correct:** Paroxetine is a Selective Serotonin Reuptake Inhibitor (SSRI), while Tranylcypromine is a non-selective Monoamine Oxidase Inhibitor (MAOI). When these two classes are combined, they cause an excessive accumulation of serotonin in the synaptic cleft. This leads to **Serotonin Syndrome**, a potentially fatal condition characterized by the "triad" of cognitive effects (confusion, agitation), autonomic hyperactivity (hyperthermia, tachycardia), and somatic effects (myoclonus, rigidity). To avoid this, a "washout period" of at least 14 days is required when switching between these drugs. **2. Why the other options are incorrect:** * **Diazepam:** This is a benzodiazepine. While SSRIs and benzodiazepines are often co-prescribed for anxiety disorders, they do not have a life-threatening pharmacodynamic interaction. * **Digoxin:** Digoxin metabolism is not significantly affected by SSRIs like paroxetine. Interactions are more common with drugs affecting P-glycoprotein or renal clearance. * **Halothane:** This is an inhaled anesthetic. While halothane can sensitize the myocardium to catecholamines, it does not have a direct, clinically significant interaction with serotonin uptake inhibitors. **High-Yield Clinical Pearls for NEET-PG:** * **Serotonin Syndrome vs. NMS:** Remember that Serotonin Syndrome presents with **hyperreflexia and myoclonus**, whereas Neuroleptic Malignant Syndrome (NMS) presents with **"lead-pipe" rigidity** and is caused by dopamine antagonists. * **The Fluoxetine Exception:** Most SSRIs require a 2-week washout period before starting an MAOI, but **Fluoxetine** requires a **5-week** washout due to its long-acting metabolite (norfluoxetine). * **Drug of Choice:** SSRIs are currently the first-line treatment for Depression, OCD, Panic Disorder, and Social Phobia.

Q: Which of the following drugs should not be administered along with levodopa?

Vitamin B complex. **Explanation:** The correct answer is **Vitamin B complex**, specifically due to the presence of **Pyridoxine (Vitamin B6)**. **1. Why Vitamin B complex is the correct answer:** Levodopa is a precursor to dopamine that must cross the blood-brain barrier (BBB) to be effective. In the periphery, the enzyme **aromatic L-amino acid decarboxylase (LAAD)** converts levodopa into dopamine. Pyridoxine acts as a vital cofactor for this enzyme. When Vitamin B6 is administered, it significantly accelerates the peripheral decarboxylation of levodopa. Since dopamine cannot cross the BBB, this leads to decreased availability of levodopa for the brain and increased peripheral side effects (like nausea and cardiac arrhythmias). **2. Why the other options are incorrect:** * **Carbidopa (A) and Benserazide (D):** These are **peripheral decarboxylase inhibitors**. They are intentionally co-administered with levodopa to prevent its peripheral breakdown, thereby increasing its CNS bioavailability and reducing side effects. * **MAO inhibitors (B):** While non-selective MAO inhibitors are contraindicated with levodopa (due to the risk of hypertensive crisis), selective **MAO-B inhibitors** (like Selegiline) are frequently used as adjuncts to levodopa therapy to prevent dopamine breakdown in the brain. **High-Yield Clinical Pearls for NEET-PG:** * **The "Pyridoxine Reversal":** This interaction only occurs when levodopa is given **alone**. If levodopa is combined with carbidopa (e.g., Sinemet), pyridoxine does *not* interfere with its action because the decarboxylase inhibitor blocks the peripheral effect of B6. * **Dietary Note:** Patients on levodopa monotherapy should be advised to avoid high-protein meals, as neutral amino acids compete with levodopa for transport across the BBB. * **Drug of Choice:** Levodopa + Carbidopa remains the gold standard for symptomatic management of Parkinson’s disease.

Q: Which of the following medications can cause oral contraceptive failure?

Rifampicin. The correct answer is **Rifampicin**. The primary mechanism behind oral contraceptive (OCP) failure is the induction of hepatic microsomal enzymes, specifically the **Cytochrome P450 (CYP3A4)** system [1]. **Why Rifampicin is correct:** Rifampicin is one of the most potent **enzyme inducers** known [4]. It increases the metabolism of the estrogenic component (Ethinylestradiol) of OCPs in the liver. This leads to significantly lower plasma concentrations of the hormones, falling below the therapeutic threshold required to suppress ovulation, thereby resulting in contraceptive failure. **Analysis of Incorrect Options:** * **Penicillin:** While some older theories suggested that broad-spectrum antibiotics might interfere with the enterohepatic circulation of estrogens by altering gut flora, clinical studies have shown that most antibiotics (including Penicillins) do not significantly reduce OCP efficacy. * **Chloroquine:** This is an antimalarial that does not possess significant enzyme-inducing properties and does not interfere with OCP metabolism. * **Metronidazole:** This is an antiprotozoal/antibacterial that is actually an **enzyme inhibitor** (specifically of CYP2C9). While it can cause a Disulfiram-like reaction with alcohol, it does not cause OCP failure. **High-Yield Clinical Pearls for NEET-PG:** * **Other Enzyme Inducers (The "GPRS Cell Phone" Mnemonic):** **G**riseofulvin, **P**henytoin, **R**ifampicin, **S**moking, **C**arbamazepine, **P**henobarbitone [2], [3]. All of these can potentially cause OCP failure [3]. * **Management:** Patients on Rifampicin should be advised to use an alternative or additional method of contraception (e.g., barrier methods) during and for 4 weeks after stopping the drug. * **Exception:** Rifabutin is a less potent inducer than Rifampicin but still requires caution.

Q: Clinically significant drug interaction occurs between pyridoxine and all the following drugs except?

Cyclosporine. **Explanation:** The core concept behind this question is the metabolic relationship between certain drugs and **Vitamin B6 (Pyridoxine)**. Pyridoxine acts as a co-factor for various decarboxylase enzymes. **Why Cyclosporine is the correct answer:** Cyclosporine is a calcineurin inhibitor used as an immunosuppressant. Its metabolism and mechanism of action do not involve pyridoxine pathways, nor does it interfere with B6 absorption or excretion. Therefore, there is no clinically significant interaction between the two. **Why the other options are incorrect:** * **Isoniazid (INH):** This is a classic interaction. INH inhibits the enzyme *pyridoxine phosphokinase*, preventing the conversion of B6 to its active form (Pyridoxal Phosphate). It also forms hydrazones with B6, leading to its excretion. This causes **peripheral neuropathy**, which is why B6 (10–50 mg/day) is co-administered with INH. * **Levodopa:** Pyridoxine is a cofactor for the enzyme *peripheral dopa-decarboxylase*. High doses of B6 accelerate the peripheral conversion of Levodopa to Dopamine. Since Dopamine cannot cross the blood-brain barrier, this **reduces the therapeutic efficacy** of Levodopa in Parkinsonism. (Note: This interaction is minimized when Levodopa is combined with Carbidopa). * **Hydralazine:** Similar to INH, this vasodilator can react with pyridoxine to form complexes, leading to a deficiency that manifests as peripheral neuritis. **High-Yield Clinical Pearls for NEET-PG:** * **Penicillamine** also causes B6 deficiency by forming a thiazolidine derivative with the vitamin. * **Oral Contraceptive Pills (OCPs)** can induce a relative B6 deficiency, sometimes linked to depression in users. * **Sideroblastic Anemia:** Pyridoxine is used to treat certain types of sideroblastic anemia as it is a cofactor for ALA synthase.

Q: Pethidine should not be given with which of the following drug classes?

MAO inhibitors. **Explanation:** The contraindication of **Pethidine (Meperidine)** with **MAO Inhibitors (MAOIs)** is a classic, high-yield pharmacological interaction. **Why MAO Inhibitors are the Correct Answer:** Pethidine acts as a weak Serotonin Reuptake Inhibitor (SRI). When combined with MAOIs (which prevent the breakdown of serotonin), it can lead to a life-threatening **Serotonin Syndrome**. This manifests as the "Excitatory Type" reaction, characterized by hyperpyrexia (very high fever), agitation, tremors, convulsions, and cardiovascular instability. Additionally, a "Depressive Type" reaction can occur, involving respiratory depression and hypotension, due to the inhibition of hepatic enzymes by MAOIs, which leads to toxic accumulation of pethidine. **Why Other Options are Incorrect:** * **Reserpine (A):** While reserpine depletes monoamines, it does not trigger the acute, fatal serotonergic storm seen with MAOIs. * **Propranolol (B) & Atenolol (C):** These are Beta-blockers. While they may have minor interactions with various drugs regarding heart rate or blood pressure, there is no absolute contraindication or major toxic syndrome associated with their co-administration with pethidine. **Clinical Pearls for NEET-PG:** * **The "Pethidine Rule":** Always screen for MAOI use (e.g., Phenelzine, Selegiline) before administering pethidine. * **Alternative:** If an opioid is needed for a patient on MAOIs, **Morphine** is generally considered safer as it lacks significant serotonergic activity. * **Metabolite Fact:** Pethidine is metabolized to **Norpethidine**, which is neurotoxic and can cause seizures, especially in patients with renal failure. * **Unique Property:** Unlike morphine, pethidine has **anticholinergic properties**, leading to side effects like tachycardia and mydriasis (dilated pupils).

Q: Which of the following antiepileptic drugs does not cause oral contraceptive pill (OCP) failure?

Valproate. ### Explanation The core pharmacological concept behind this question is **Cytochrome P450 (CYP450) enzyme induction**. **1. Why Valproate is the Correct Answer:** Valproate is a potent **enzyme inhibitor**, not an inducer. Unlike many traditional antiepileptics, it does not increase the metabolism of steroid hormones. Therefore, it does not reduce the plasma concentration of estrogen or progesterone, meaning it **does not cause OCP failure**. It is considered safe to co-administer with hormonal contraceptives regarding contraceptive efficacy. **2. Why the Other Options are Incorrect:** * **Phenytoin, Carbamazepine, and Phenobarbitone** are classic, potent **CYP450 enzyme inducers** (specifically CYP3A4). * These drugs increase the rate of hepatic metabolism of the estrogenic and progestogenic components of the OCP [1]. * This leads to sub-therapeutic levels of the hormones, resulting in "breakthrough ovulation" and unintended pregnancy (OCP failure) [2]. **3. High-Yield Clinical Pearls for NEET-PG:** * **The "Big Four" Inducers:** Remember the mnemonic **GPPRS** (Griseofulvin, Phenytoin, Phenobarbitone, Rifampicin, St. John's Wort) and **Carbamazepine**. These all risk OCP failure. * **Newer AEDs:** Lamotrigine, Oxcarbazepine, and Topiramate (at high doses) can also decrease OCP efficacy. * **Safe Alternatives:** Levetiracetam, Valproate, Gabapentin, and Vigabatrin are generally considered "non-inducers" and do not interfere with OCPs. * **Clinical Management:** If an enzyme inducer must be used, patients are advised to use an alternative form of contraception (e.g., Barrier methods or IUD) or a high-dose estrogen pill (>50μg), though the latter is less preferred.

Question 1

Renal excretion of lithium is reduced by which of the following medications?

Accepted Answer

All of the above

Answer

Furosemide

Answer

Indomethacin

Answer

Hydrochlorothiazide

Question 2

Following a myocardial infarct, a patient is stabilized on warfarin, with the dose adjusted to give a prothrombin time of 22 seconds. Which one of the following statements regarding potential drug interactions in this patient is accurate?

Accepted Answer

Antibacterial sulfonamides may enhance the effects of warfarin.

Answer

Cholestyramine will increase prothrombin time.

Answer

Cimetidine is likely to decrease prothrombin time.

Answer

Vitamin K would restore prothrombin time to normal within 30 minutes.

Question 3

All the following Antiepileptics are inducers except?

Accepted Answer

Valproate

Answer

Phenobarbitone

Answer

Phenytoin

Answer

Rifampicin

Question 4

Inhibitors of serotonin (5-HT) uptake, such as paroxetine, can cause clinically significant drug interactions with which of the following?

Accepted Answer

Tranylcypromine

Answer

Diazepam

Answer

Digoxin

Answer

Halothane

Question 5

Which of the following drugs should not be administered along with levodopa?

Accepted Answer

Vitamin B complex

Answer

Carbidopa

Answer

MAO inhibitors

Answer

Benserazide

Question 6

Which of the following medications can cause oral contraceptive failure?

Accepted Answer

Rifampicin

Answer

Penicillin

Answer

Chloroquine

Answer

Metronidazole

Question 7

Dextromethorphan should not be administered concurrently with which class of drugs?

Accepted Answer

Monoamine Oxidase Inhibitors (MAOIs)

Answer

Selective Serotonin Reuptake Inhibitors (SSRIs)

Answer

Atropine

Answer

Paracetamol

Question 8

Clinically significant drug interaction occurs between pyridoxine and all the following drugs except?

Accepted Answer

Cyclosporine

Answer

Isoniazid

Answer

Levodopa

Answer

Hydralazine

Question 9

Pethidine should not be given with which of the following drug classes?

Accepted Answer

MAO inhibitors

Answer

Reserpine

Answer

Propranolol

Answer

Atenolol

Question 10

Which of the following antiepileptic drugs does not cause oral contraceptive pill (OCP) failure?

Accepted Answer

Valproate

Answer

Phenytoin

Answer

Carbamazepine

Answer

Phenobarbitone

Drug Interactions — MCQs

Drug Interactions — MCQs

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