Question – A 60-year-old male suffering from parkinsonism and was stable on Levodopa. He was prescribed pyridoxine. After one month his condition deteriorated.

Explanation- Pyridoxine promote formation of dopamine in peripheral tissue (from levodopa) hence less levodopa is available to act in brain (abolition of therapeutic effect seen)

To prevent this, levodopa is used in combination with Carbidopa (Peripheral decarboxylase inhibitor), and avoid concurrent use of pyridoxine with levodopa.

Question – A 60-year-old male suffering from bipolar disorder stable on lithium from one year. He was diagnosed with Congestive heart failure and prescribed chlorothiazide, after one month he is admitted with complain of blurred vision, nystagmus, ataxia, motor incoordination.

Explanation- Diuretics (Thiazide, Furosemide) causes proximal tubular reabsorption of Na+ as well as Li+ and this causes reduced excretion of lithium, due to rise in plasma level lithium toxicity occur.

To overcome this, reduce dose of Li+ and monitor plasma level of Li+ or avoid concurrent use.

Potassium sparing diuretics can be also used because they cause milder retention of Li+

Question – A 50-year-old male, Hypertensive, taking metoprolol. He was prescribed Diclofenac for joint pain. Two months later his BP was raised.

Explanation- Indomethacin and other NSAIDS attenuate the antihypertensive effect of Beta blockers.

To prevent this, avoid concurrent use of NSAIDS along with beta blockers for long term.

Question – A 50-year-old male, suffering from Congestive heart failure, taking metoprolol. He was prescribed verapamil accidently. Next day days he was admitted with severe bradycardia.

Explanation- This is a case of Additive depression of SA node and AV conduction (both drugs have depressive effect on SA node and AV condition, hence severe bradycardia)

So, to prevent this avoid concurrent use of both drugs. Another drug which can lead to similar situation is Digitalis.

Question – A 50-year-old male, suffering from epilepsy, stable on phenytoin. He was prescribed antitubercular therapy after ten days he was admitted with seizures.

Explanation- This is a case of taking an enzyme inhibitor drug (Isoniazid). This enzyme inhibitor causes inhibition of metabolism of phenytoin causing potentiation of effect lead to toxicity of phenytoin like twitching eye movements, slurred speech, loss of balance, tremor, muscle stiffness, nausea, vomiting, fainting, shallow breathing, convulsions etc.

To avoid this type of interaction, avoid concurrent use of these drugs or readjust (reduce) the dose of phenytoin.

Example of Enzyme inhibitor drugs are – Valproate, Erythromycin, Ciprofloxacin, Cimetidine, Ketoconazole, Itraconazole etc. 

Question – A 40-year-old male, stable on warfarin, has prescribed fluconazole for fungal infection and after a week he was admitted in emergency with intracranial bleeding.

Explanation- This is a case of taking an enzyme inhibitor drug (Fluconazole, inhibit metabolism of CYP3A4). This enzyme inhibitor causes inhibition of metabolism of warfarin causing potentiation of effect lead to bleeding.

To avoid this type of interaction avoid concurrent use of these drugs or readjust (reduce) the dose of warfarin.

Example of Enzyme inhibitor drugs are – Erythromycin, Clarithromycin, Ketoconazole, Itraconazole etc.  

Question – A 29-year-old female, Taking oral contraceptive pills (OCPs) as birth control measure from last three years.  She was diagnosed with tuberculosis and prescribed rifampicin and become pregnant after two months.

Explanation- This is a case of taking an enzyme inducer drug (Rifampicin). This enzyme inducer causes induction of metabolism of OCP and lead to pregnancy.

In this type of situation where we have to give rifampicin, we advise the patient to shift for other methods of contraception (like condoms etc.) to avoid this type of interaction.

Example of Enzyme inducer drugs – Griseofulvin, Phenytoin, Phenobarbitone, Carbamazepine, Rifampicin etc.  (Note that chronic smoking also act as enzyme inducer)

Question – A 29-year-old male, chronic alcoholic, felt abdominal discomfort and directly take Tab. Metronidazole from medical store and consumed alcohol at night. Afterwards he felt severe distressing problems like headache, nausea, vomiting, sweating, vertigo, blurred vision, hypotension, dizziness, mental confusion and admitted in the emergency department.

Explanation- This is a case of drug interaction between Metronidazole and Alcohol.

This type of reactions occurs when alcohol is consumed with drugs which inhibit aldehyde dehydrogenase enzyme. The inhibition of this enzyme leads to accumulation of acetaldehyde in body leads to mentioned distressing symptoms.

Disulfiram, Tinidazole, cefoperazone, Griseofulvin are some drugs which leads to the mentioned reactions commonly known as “Disulfiram like reactions”

So always warn patient strictly not to consume alcohol specially when using these drugs.

Question – A 55-year-old male taking enalapril 5 mg tablet for hypertensin from last three years continuously. Recently he was prescribed spironolactone for Congestive Heart Failure and after 5 days he was admitted in emergency with complaints of palpitation and restlessness .

Explanation- This is a case of drug interaction between Enalapril and Spironolactone.

Enalapril causes Hyperkalaemia, Spironolactone is a K+ Sparing diuretic so severe hyperkalaemia can occur, leads cardiac condition abnormalities.

K+ supplements, KCl, other ACE Inhibitors (Lisinopril, Ramipril), ARB blockers (Losartan, telmisartan, valsartan) can lead to similar situation if combined with Spironolactone (K+ sparing diuretics)

Question – A 19-year-old female was taking iron tablets for the anaemia from three months, on investigation there was no significant improvement in anaemia. After detailed history she told that due to bad taste of iron tablets she was consuming them with milk.

She was later directed not to take iron tablets with milk, tea or calcium tablets (at same time).

Explanation- This is a case of drug interaction between iron and calcium.

If taken together the absorption of iron is decreased due to formation of complexes between iron and calcium (present in milk)

That’s why it is advisable to wait at least two hours before consuming milk, tea or calcium tablets after iron tablets or Vise versa.

Question - A Male patient age 64 years, taking nitrate for angina. He consumes Tab. Sildenafil for erectile dysfunction and admitted to Emergency department with tachycardia and chest pain.

Explanation- This is a typical case of drug interaction between Nitrates and Phosphodiesterase (PDE-5) inhibitors like Sildenafil, Tadalafil, vardenafil, avanafil etc.

Use of Nitrates (GTN) causes hypotension and Phosphodiesterase (PDE 5) inhibitors aggravate this hypotension. This sudden severe hypotension can lead to MI and deaths (because nitrates increase generation of cGMP, while sildenafil prevents its degradation by inhibiting PDE-5)

That’s why Concurrent use of both drugs is contraindicated.

So, avoid such combination and use other therapeutic options for erectile dysfunction patients.