Keflex (Cephalexin) Does Not Contain Crustacean Components
No, Keflex (cephalexin) capsules do not contain any crustacean components based on the FDA drug label information.
Composition of Cephalexin
According to the FDA drug label, cephalexin is a semisynthetic cephalosporin antibiotic with the chemical structure 7-(D-α-amino-α-phenylacetamido)-3-methyl-3-cephem-4-carboxylic acid, monohydrate 1. The composition of Keflex capsules includes:
- Active ingredient: Cephalexin monohydrate
- Inactive ingredients: magnesium stearate, silicon dioxide, and sodium starch glycolate
- Capsule shell components: black iron oxide, D&C Yellow #10 Aluminum Lake, FD&C Blue #1 Aluminum Lake, FD&C Blue #2 Aluminum Lake, FD&C Red #40 Aluminum Lake, gelatin, pharmaceutical glaze modified in SD-45, silicon dioxide or carboxymethylcellulose sodium, sodium lauryl sulfate, titanium dioxide 1
Chemical Structure and Origin
Cephalexin is derived from cephalosporin C, which is produced by the fungus Acremonium (formerly known as Cephalosporium). It is a semisynthetic antibiotic, meaning it is created through chemical modification of naturally occurring compounds, but does not contain any animal-derived components from crustaceans 1, 2.
The nucleus of cephalexin is related to that of other cephalosporin antibiotics, featuring a bicyclic ring system that differs from penicillins. Specifically, cephalexin has:
- A D-phenylglycyl group at the 7-amino position
- An unsubstituted methyl group at the 3-position 1
Allergic Considerations
While cephalexin does not contain crustacean components, it's important to note that:
Patients with penicillin allergies may have cross-reactivity with cephalosporins, though this is primarily due to similar beta-lactam ring structures and side chains, not due to any crustacean content 3.
The cross-reactivity between penicillins and cephalosporins is now understood to be much lower (approximately 2%) than previously thought (earlier estimates of 8-10%) 3.
Cross-reactivity is primarily based on R1 side chains, with identical side chains posing the highest risk in patients with IgE-mediated allergies 3.
Clinical Implications
For patients concerned about crustacean allergies:
- Cephalexin can be safely used in patients with shellfish or crustacean allergies as there is no relationship between these allergies and reactions to cephalosporins 3, 1.
- The common misconception about shellfish allergy and antibiotic allergies stems from confusion about the term "allergy" rather than any shared components.
For patients with penicillin allergies considering cephalexin:
- The decision should be based on the nature of the previous reaction and the similarity of side chains, not concerns about crustacean content 3.
Summary
Keflex (cephalexin) is a semisynthetic antibiotic derived from fungal sources and modified chemically. The FDA drug label clearly lists all components, none of which are derived from or contain crustacean materials 1. Patients with shellfish or crustacean allergies can safely take cephalexin without concern for allergic reactions based on those specific allergies.