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Side Effects


The most common adverse reactions ( ≥ 1%) reported in clinical trials when all three components of this therapy were given concomitantly are listed in Table 1 below. The majority of the adverse reactions were related to the gastrointestinal tract, were reversible, and infrequently led to discontinuation of therapy.

Table 1: Incidence of Adverse Reactions Reported in Clinical Trials ( ≥ 1%)†

Adverse Reactions Bismuth Subsalicylate, Metronidazole, and Tetracycline ‡
(N = 266)
% Patients
Nausea 12
Diarrhea 6.8
Abdominal Pain 6.8
Melena 3
Upper Respiratory Infection 2.3
Constipation 1.9
Anorexia 1.5
Asthenia 1.5
Vomiting 1.5
Discolored Tongue* 1.5
Headache 1.5
Dyspepsia 1.5
Dizziness 1.5
Stool Abnormality** 1.1
Duodenal Ulcer 1.1
Sinusitis 1.1
Taste Perversion*** 1.1
Flatulence 1.1
GI Hemorrhage 1.1
Pain 1.1
Insomnia 1.1
Anal Discomfort 1.1
Paresthesia 1.1
* darkening of the tongue
** black or dark stools
*** metallic taste
† Includes reactions reported at ≥ 1% in patients taking bismuth subsalicylate, metronidazole, and tetracycline in Studies 1, 2, 3, and 4 (see Clinical Studies).
‡ In Studies 1, 2, and 3(N = 197), most patients were on concomitant acid suppression therapy.

The additional adverse reactions ( < 1%) reported in clinical trials when all three components of this therapy were given concomitantly are listed below and divided by body system:

Gastrointestinal: dry mouth, dysphagia, eructation, GI monilia, glossitis, intestinal obstruction, rectal hemorrhage, stomatitis

Skin: acne, ecchymosis, photosensitivity reaction (see WARNINGS), pruritus, rash

Cardiovascular: cerebral ischemia, chest pain, hypertension, myocardial infarction

CNS: nervousness, somnolence

Musculoskeletal: arthritis, rheumatoid arthritis, tendonitis

Metabolic:SGOT increase, SGPT increase

Urogenital: urinary tract infection

Other: conjunctivitis, flu syndrome, infection, malaise, neoplasm, rhinitis, syncope, tooth disorder

Other Important Adverse Reactions from Labeling for the Individual Components of HELIDAC Therapy


Blood and Lymphatic system disorders: reversible neutropenia (leucopenia) in cases of prolonged treatment; rarely reversible thrombocytopenia however no persistent haematological abnormalities attributable to metronidazole have been observed. (See PRECAUTIONS)

Cardiac disorders: Flattening of the T-wave may be seen in electrocardiographic tracings.

Gastrointestinal disorders: Furry tongue, glossitis, stomatitis; these may be associated with a sudden overgrowth of candida which may occur during therapy; epigastric distress (See PRECAUTIONS)

Immune system disorders: Urticaria, erythematous rash, Stevens-Johnson Syndrome, toxic epidermal necrolysis, flushing, nasal congestion, dryness of the mouth (or vagina or vulva) and fever (See CONTRAINDICATIONS)

Metabolism and nutrition disorders: Cases of pancreatitis have been reported, which abated on withdrawal of the drug, have been reported.

Nervous system disorders: The most serious adverse reactions reported in patients treated with metronidazole have been convulsive seizures, encephalopathy, aseptic meningitis, optic and peripheral neuropathy, the latter characterized mainly by numbness or paresthesia of an extremity. In addition, patients have reported syncope, vertigo, incoordination, ataxia, confusion, dysarthria, irritability, depression, weakness, and insomnia. (See WARNINGS)

Renal disorders: Dysuria, cystitis, polyuria, incontinence, and a sense of pelvic pressure. Instances of darkened urine have been reported by approximately one patient in 100,000. Although the pigment which is probably responsible for this phenomenon has not been positively identified, it is almost certainly a metabolite of metronidazole and seems to have no clinical significance.

Other: Proliferation of Candida in the vagina, dyspareunia, decrease of libido, proctitis, and fleeting joint pains sometimes resembling “serum sickness.” Crohn's disease patients are known to have an increased incidence of gastrointestinal and certain extraintestinal cancers. There have been some reports in the medical literature of breast and colon cancer in Crohn's disease patients who have been treated with metronidazole at high doses for extended periods of time. A cause and effect relationship has not been established.

Tetracycline Hydrochloride

Blood and lymphatic system disorders: Hemolytic anemia, thrombocytopenia, thrombocytopenic purpura, neutropenia, and eosinophilia.

Gastrointestinal disorders: Anorexia, nausea, epigastric distress, glossitis, black hairy tongue, dysphagia, enterocolitis, and inflammatory lesions (with monilial overgrowth) in the anogenital region. Rare instances of esophagitis and esophageal ulceration have been reported in patients taking the tetracycline-class antibiotics in capsule and tablet form. Most of the patients who experienced esophageal irritation took the medication immediately before going to bed. Permanent discoloration of teeth may be caused when tetracycline is used during tooth development. Enamel hypoplasia has also been reported. (See WARNINGS)

Hypersensitivity: urticaria, angioneurotic edema, anaphylaxis, anaphylactoid purpura, pericarditis, exacerbation of systemic lupus erythematosus and serum sickness-like reactions, as fever, rash, and arthralgia.

Liver disorders: Hepatotoxicity and liver failure have been observed in patients receiving large doses of tetracycline and in tetracycline-treated patients with renal impairment. Increases in liver enzymes and hepatic toxicity have been reported rarely.

Nervous system disorders: Pseudotumor cerebri (benign intracranial hypertension) in adults and bulging fontanels in infants, Visual disturbances. Tinnitus and myasthenic syndrome have been reported rarely.

Renal and urinary disorders: Rise in BUN has been reported and is possibly dose related (See CONTRAINDICATIONS)

Skin and subcutaneous tissue disorders: Maculopapular and erythematous rashes have been reported. Stevens-Johnson syndrome, toxic epidermal necrolysis, and erythema multiforme have been reported. Onycholysis, discoloration of the nails, exfoliative dermatitis, and photosensitivity have been rarely reported. (See PRECAUTIONS)

Other: When given over prolonged periods, tetracyclines have been reported to produce brown-black microscopic discoloration of thyroid glands. No abnormalities of thyroid function studies are known to occur.

Read the Helidac (bismuth subsalicylate) Side Effects Center for a complete guide to possible side effects



Do not administer methoxyflurane to patients taking HELIDAC Therapy. The concurrent use of tetracycline hydrochloride, a component of HELIDAC Therapy, with methoxyflurane has been reported to result in fatal renal toxicity. (See CONTRAINDICATIONS)


Psychotic reactions have been reported in alcoholic patients who are using metronidazole and disulfiram concurrently. Metronidazole should not be given to patients who have taken disulfiram within the last 2 weeks. (See CONTRAINDICATIONS)


Consumption of alcoholic beverages or administration of other products containing propylene glycol during treatment with HELIDAC Therapy and for at least 3 days afterwards may cause a disulfiram-like reaction (abdominal cramps, nausea, vomiting, headaches, and flushing) due to the interaction between alcohol or propylene glycol and metronidazole, a component of HELIDAC Therapy. Discontinue alcoholic beverage or other products containing propylene glycol during and for at least 3 days after therapy with HELIDAC Therapy (See CONTRAINDICATIONS)

Oral Contraceptives

Concurrent use of HELIDAC Therapy with oral contraceptive may make oral contraceptives less effective due to an interaction with the tetracycline component of HELIDAC Therapy. Breakthrough bleeding has been reported. Women of child-bearing potential should use a different or additional form of contraception while taking HELIDAC Therapy. Women who become pregnant while on the HELIDAC Therapy should be advised to notify their prescriber immediately.


HELIDAC Therapy may alter the anticoagulant effects of warfarin and other oral coumarin anticoagulants. Salicylates may cause an increased risk of bleeding when administered with anticoagulant therapy. Metronidazole has been reported to potentiate the anticoagulant effect of warfarin, and other oral coumarin anticoagulants, resulting in a prolongation of prothrombin time. Tetracycline has been shown to depress plasma prothrombin activity. Prothrombin time, International Normalized Ratio (INR), or other suitable anticoagulation tests should be closely monitored if HELIDAC Therapy is administered concomitantly with warfarin. Patients should also be monitored for evidence of bleeding.


In patients stabilized on relatively high doses of lithium, short-term use of HELIDAC Therapy may cause elevation of serum lithium concentrations and signs of lithium toxicity due to the interaction between metronidazole and lithium. Serum lithium and serum creatinine concentrations should be monitored several days after beginning treatment with HELIDAC to detect any increase that may precede clinical symptoms of lithium toxicity.

Antacids, Multivitamins, or Dairy Products

The absorption of HELIDAC Therapy may be reduced if administered with antacids containing aluminium, calcium, or magnesium; preparations containing iron, zinc, or sodium bicarbonate; or milk or dairy products due to the interaction between these products and tetracycline. These products should not be consumed concomitantly with HELIDAC Therapy. However, the clinical significance of reduced tetracycline systemic exposure is unknown as the relative contribution of systemic versus local antimicrobial activity against Helicobacter pylori has not been established.

Antidiabetic agents, Aspirin, Probenecid, or Sulfinpyrazone

Caution is advised in the administration of bismuth subsalicylate to patients taking medication for diabetes (possible enhanced hypoglycemic effect when given with salicylates) or patients taking aspirin (may increase the risk of salicylate toxicity), probenecid, or sulfinpyrazone (uricosuric effects of probenecid or sulfinpyrazone may be decreased).


Bacteriostatic drugs, such as the tetracycline class of antibiotics, may interfere with the bactericidal action of penicillin; therefore pencillin should not be used concomitantly with HELIDAC Therapy.

Inhibitors of CYP450 liver enzymes

The simultaneous administration of HELIDAC Therapy and drugs that inhibit microsomal liver enzymes, such as cimetidine, may result in a prolonged half-life and decreased plasma clearance of metronidazole.

Inducers of CYP450 liver enzymes

The simultaneous administration of HELIDAC Therapy and drugs that induce microsomal liver enzymes, such as phenytoin or phenobarbital, may accelerate the elimination of metronidazole, resulting in reduced plasma concentrations of metronidazole. Impaired clearance of phenytoin has also been reported in this situation. Monitor phenytoin concentrations during treatment with HELIDAC Therapy.

Drug/Laboratory Test Interactions

Bismuth absorbs x-rays and may interfere with x-ray diagnostic procedures of the gastrointestinal tract.

Bismuth subsalicylate may cause a temporary and harmless darkening of the stool. However, this does not interfere with standard tests for occult blood.

Metronidazole may interfere with certain types of determinations of serum chemistry values, such as aspartate aminotransferase (AST, SGOT), alanine aminotransferase (ALT, SGPT), lactate dehydrogenase (LDH), triglycerides, and hexokinase glucose. Values of zero may be observed. All of the assays in which interference has been reported involve enzymatic coupling of the assay to oxidation-reduction of nicotinamide (NAD+ ⇔ NADH). Interference is due to the similarity in absorbance peaks of NADH (340 nm) and metronidazole (322 nm) at pH 7.

Read the Helidac Drug Interactions Center for a complete guide to possible interactions

This monograph has been modified to include the generic and brand name in many instances.

Last reviewed on RxList: 6/29/2012

Side Effects

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