Author(s): Ilan Youngster, Elizabeth Barnett
On This Page
- Vaccine–Vaccine Interactions
- Antimalarial Drugs
- Drugs Used for Travel to High Elevations
- HIV Medications
During pretravel consultations, travel health providers must consider potential interactions between vaccines and medications, including those already taken by the traveler. A study by S. Steinlauf et al. identified potential drug–drug interactions with travel-related medications in 45% of travelers taking medications for chronic conditions; 3.5% of these interactions were potentially serious.
Vaccine–Vaccine Interactions
Most common vaccines can be given safely and effectively at the same visit, at separate injection sites, without impairing antibody response or increasing rates of adverse reactions. However, certain vaccines, including pneumococcal and meningococcal vaccines and live virus vaccines, require appropriate spacing; further information about vaccine–vaccine interactions is found in Sec. 2, Ch. 3, .
Travel Vaccines & Drugs
Live Attenuated Oral Typhoid & Cholera Vaccines
Live attenuated vaccines generally should be avoided in immunocompromised travelers, including those taking antimetabolites, calcineurin inhibitors, cytotoxic agents, immunomodulators, and high-dose steroids (see Table 3-04).
Antimalarial Drugs
Chloroquine and atovaquone-proguanil at doses used for malaria chemoprophylaxis can be given concurrently with oral typhoid vaccine. Data from an older formulation of the CVD 103-HgR oral cholera vaccine suggest that the immune response to the vaccine might be diminished when given concomitantly with chloroquine. Administer live attenuated oral cholera vaccine ≥10 days before beginning antimalarial prophylaxis with chloroquine. A study in children using oral cholera vaccine suggested no decrease in immunogenicity when given with atovaquone-proguanil.
Antimicrobial Agents
Antimicrobial agents can be active against the vaccine strains in the oral typhoid and cholera vaccines and might prevent adequate immune response to these vaccines. Therefore, delay vaccination with oral typhoid vaccine by >72 hours and delay oral cholera vaccine by >14 days after administration of antimicrobial agents. Parenteral typhoid vaccine is an alternative to the oral typhoid vaccine for travelers who have recently received antibiotics.
Rabies Vaccine
Concomitant use of chloroquine can reduce the antibody response to intradermal rabies vaccine administered as a preexposure vaccination. Use the intramuscular route for people taking chloroquine concurrently. Intradermal administration of rabies vaccine is not currently approved for use in the United States (see Sec. 5, Part 2, Ch. 19, . . . perspectives: Rabies Immunization).
Antimalarial Drugs
Any time a new medication is prescribed, including antimalarial drugs, check for known or possible drug interactions (see Table 2-05) and inform the traveler of potential risks. Online clinical decision support tools (e.g., Micromedex) provide searchable databases of drug interactions.
Atovaquone-Proguanil
Antibiotics
Rifabutin, rifampin, and tetracycline might reduce plasma concentrations of atovaquone and should not be used concurrently with atovaquone-proguanil.
Anticoagulants
Patients on warfarin might need to reduce their anticoagulant dose or monitor their prothrombin time more closely while taking atovaquone-proguanil, although coadministration of these drugs is not contraindicated. The use of novel oral anticoagulants, including dabigatran, rivaroxaban, and apixaban, is not expected to cause significant interactions, and their use has been suggested as an alternative for patients in need of anticoagulation.
Antiemetics
Metoclopramide can reduce bioavailability of atovaquone; unless no other antiemetics are available, this antiemetic should not be used to treat vomiting associated with the use of atovaquone at treatment doses.
Antihistamines
Travelers taking atovaquone-proguanil for malaria prophylaxis should avoid using cimetidine (an H2 receptor antagonist) because this medication interferes with proguanil metabolism.
HIV Medications
Atovaquone-proguanil might interact with the antiretroviral protease inhibitors atazanavir, darunavir, indinavir, lopinavir, and ritonavir, or the nonnucleoside reverse transcriptase inhibitors (NNRTIs) efavirenz, etravirine, and nevirapine, resulting in decreased levels of atovaquone-proguanil. For travelers taking any of these medications, consider alternative malaria chemoprophylaxis.
Selective Serotonin Reuptake Inhibitors
Fluvoxamine interferes with the metabolism of proguanil; consider an alternative antimalarial prophylaxis to atovaquone-proguanil for travelers taking this selective serotonin reuptake inhibitor (SSRI).
Chloroquine
Antacids & Antidiarrheals
Chloroquine absorption might be reduced by antacids or kaolin; travelers should wait ≥4 hours between doses of these medications.
Antibiotics
Chloroquine inhibits bioavailability of ampicillin, and travelers should wait ≥2 hours between doses of these medications. Chloroquine should not be coadministered with either clarithromycin or erythromycin; azithromycin is a suggested alternative. Chloroquine also reportedly decreases the bioavailability of ciprofloxacin.
Antihistamines
Concomitant use of cimetidine and chloroquine should be avoided because cimetidine can inhibit the metabolism of chloroquine and increase drug levels.
CYP2D6 Enzyme Substrates
Chloroquine is a CYP2D6 enzyme inhibitor. Monitor patients taking chloroquine concomitantly with other substrates of this enzyme (e.g., flecainide, fluoxetine, metoprolol, paroxetine, propranolol) for side effects.
CYP3A4 Enzyme Inhibitors
CYP3A4 inhibitors (e.g., erythromycin, ketoconazole, ritonavir) can increase chloroquine levels; concomitant use should be avoided.
Digoxin
Chloroquine can increase digoxin levels; additional monitoring is warranted.
Immunosuppressants
Chloroquine decreases the bioavailability of methotrexate. Chloroquine also can cause increased levels of calcineurin inhibitors; use caution when prescribing chloroquine to travelers taking these agents.
QT-Prolonging Agents
Avoid prescribing chloroquine to anyone taking other QT-prolonging agents (e.g., amiodarone, lumefantrine, sotalol); when taken in combination, chloroquine might increase the risk for prolonged QTc interval. In addition, the antiretroviral rilpivirine has also been shown to prolong QTc, and clinicians should avoid coadministration with chloroquine.
Doxycycline
Antacids, Bismuth Subsalicylate, Iron
Absorption of tetracyclines might be impaired by aluminum-, calcium-, or magnesium-containing antacids, bismuth subsalicylate, and preparations containing iron; advise patients not to take these preparations within 3 hours of taking doxycycline.
Antibiotics
Doxycycline can interfere with the bactericidal activity of penicillin; thus, in general, clinicians should not prescribe these drugs together. Coadministration of doxycycline with rifabutin or rifampin can lower doxycycline levels; monitor doxycycline efficacy closely or consider alternative therapy.
Anticoagulants
Patients on warfarin might need to reduce their anticoagulant dose while taking doxycycline because of its ability to depress plasma prothrombin activity.
Anticonvulsants
Barbiturates, carbamazepine, and phenytoin can decrease the half-life of doxycycline.
Antiretrovirals
Doxycycline has no known interaction with antiretroviral agents.
Immunosuppressants
Concurrent use of doxycycline and calcineurin inhibitors or mTOR inhibitors (sirolimus) can cause increased levels of these immunosuppressant drugs.
Mefloquine
Mefloquine can interact with several categories of drugs, including anticonvulsants, other antimalarial drugs, and drugs that alter cardiac conduction.
Anticonvulsants
Mefloquine can lower plasma levels of several anticonvulsant medications, including carbamazepine, phenobarbital, phenytoin, and valproic acid; avoid concurrent use of mefloquine with these agents.
Antimalarial Drugs
Mefloquine is associated with increased toxicities of the antimalarial drug lumefantrine, which is available in the United States in fixed combination to treat people with uncomplicated Plasmodium falciparum malaria. The combination of mefloquine and lumefantrine can cause potentially fatal QTc interval prolongation. Lumefantrine should therefore be avoided or used with caution in patients taking mefloquine prophylaxis.
CYP3A4 Enzyme Inducers
CYP3A4 inducers include medications used to treat HIV or HIV-associated infections (e.g., efavirenz, etravirine, nevirapine, rifabutin) and tuberculosis (rifampin). St. John’s wort and glucocorticoids are also CYP3A4 inducers. All these drugs (rifabutin and rifampin, in particular) can decrease plasma concentrations of mefloquine, thereby reducing its efficacy as an antimalarial drug.
CYP3A4 Enzyme Inhibitors
Potent CYP3A4 inhibitors (e.g., antiretroviral protease inhibitors, atazanavir, cobicistat [available in combination with elvitegravir], darunavir, lopinavir, ritonavir, saquinavir); azole antifungals (itraconazole, ketoconazole, posaconazole, voriconazole); macrolide antibiotics (azithromycin, clarithromycin, erythromycin); and SSRIs (fluoxetine, fluvoxamine, sertraline), can increase levels of mefloquine and thus increase the risk for QT prolongation.
Although no conclusive data are available regarding coadministration of mefloquine and other drugs that can affect cardiac conduction, avoid mefloquine use, or use it with caution, in patients taking antiarrhythmic or β-blocking agents, antihistamines (H1 receptor antagonists), calcium channel receptor antagonists, phenothiazines, SSRIs, or tricyclic antidepressants.
Immunosuppressants
Concomitant use of mefloquine can cause increased levels of calcineurin inhibitors and mTOR inhibitors (cyclosporine A, sirolimus, tacrolimus).
Anti-Hepatitis C Virus Protease Inhibitors
Avoid concurrent use of mefloquine and direct-acting protease inhibitors (boceprevir and telaprevir) used to treat hepatitis C. Newer direct-acting protease inhibitors (grazoprevir, paritaprevir, simeprevir) are believed to be associated with fewer drug–drug interactions, but safety data are lacking; consider alternatives to mefloquine pending additional data.
Psychiatric Medications
Avoid prescribing mefloquine to travelers with a history of mood disorders or psychiatric disease; this information is included in the US Food and Drug Administration boxed warning for mefloquine.
Drugs Used to Treat Travelers’ Diarrhea
Antimicrobials commonly prescribed as treatment for travelers’ diarrhea have the potential for interacting with several different classes of drugs (Table 2-06). As mentioned previously, online clinical decision support tools provide searchable databases that can help identify interactions with medications a person may already be taking.
Azithromycin
Anticoagulants
Increased anticoagulant effects have been noted when azithromycin is used with warfarin; monitor prothrombin time for people taking these drugs concomitantly.
Antimalarial Drugs
Because additive QTc prolongation can occur when azithromycin is used with the antimalarial artemether, avoid concomitant therapy.
HIV Medications
Drug interactions have been reported with the macrolide antibiotics, clarithromycin and erythromycin; antiretroviral protease inhibitors; and the NNRTIs, efavirenz and nevirapine. Concomitant use of azithromycin and these drugs can increase the risk of QTc prolongation, but a short treatment course is not contraindicated for those without an underlying cardiac abnormality. When azithromycin is used with the protease inhibitor nelfinavir, advise patients about possible drug interactions.
Immunosuppressants
Concurrent use of macrolides with calcineurin inhibitors can cause increased levels of drugs belonging to this class of immunosuppressants.
Fluoroquinolones
Antacids
Concurrent administration of ciprofloxacin and antacids that contain magnesium or aluminum hydroxide can reduce bioavailability of ciprofloxacin.
Anticoagulants
An increase in the international normalized ratio (INR) has been reported when levofloxacin and warfarin are used concurrently.
Asthma Medication
Ciprofloxacin decreases clearance of theophylline and caffeine; clinicians should monitor theophylline levels when ciprofloxacin is used concurrently.
Immunosuppresants
Fluoroquinolones can increase levels of calcineurin inhibitors, and doses should be adjusted for renal function.
Others
Sildenafil should not be used by patients taking ciprofloxacin; concomitant use is associated with increased rates of adverse effects. Ciprofloxacin and other fluoroquinolones should not be used in patients taking tizanidine.
Rifamycins
Rifamycin SV
No clinical drug interactions have been studied. Because of minimal systemic rifamycin concentrations observed after the recommended dose, clinically relevant drug interactions are not expected.
Rifaximin
Rifaximin is not absorbed in appreciable amounts by intact bowel, and no clinically significant drug interactions have been reported to date with rifaximin except for minor changes in INR when used concurrently with warfarin.
Drugs Used for Travel to High Elevations
Before prescribing the carbonic anhydrase inhibitor, acetazolamide, to those planning high elevation travel, carefully review with them the complete list of medications they are already taking (Table 2-07).
Acetazolamide
Acetaminophen & Diclofenac Sodium
Acetaminophen and diclofenac sodium form complex bonds with acetazolamide in the stomach’s acidic environment, impairing absorption. Neither agent should be taken within 30 minutes of acetazolamide. Patients taking acetazolamide also can experience decreased excretion of anticholinergics, dextroamphetamine, ephedrine, mecamylamine, mexiletine, and quinidine.
Anticonvulsants
Acetazolamide should not be given to patients taking the anticonvulsant topiramate because concurrent use is associated with toxicity.
Barbiturates & Salicylates
Acetazolamide causes alkaline urine, which can increase the rate of excretion of barbiturates and salicylates and could cause salicylate toxicity, particularly in patients taking a high dose of aspirin.
Corticosteroids
Hypokalemia caused by corticosteroids could occur when used concurrently with acetazolamide.
Diabetes Medications
Use caution when concurrently administering metformin and acetazolamide because of increased risk for lactic acidosis.
Immunosuppressants
Monitor cyclosporine, sirolimus, and tacrolimus more closely when given with acetazolamide.
Dexamethasone
Using dexamethasone to treat altitude illness can be lifesaving. Dexamethasone interacts with several classes of drugs, however, including: anticholinesterases, anticoagulants, digitalis preparations, hypoglycemic agents, isoniazid, macrolide antibiotics, oral contraceptives, and phenytoin.
