An auntie told me she reacts to both some older PABA-containing sunscreen brands and to the benzocaine lozenges you can buy at pharmacies. Are PABA-containing sunscreens actually related to ester local anaesthetic allergies, and why do these reactions happen?

When you hear the acronym PABA, you might think vaguely of “old-fashioned sunscreens.” But PABA is also a structural motif in ester-type local anaesthetics: a fact that connects dermatology, pharmacology, and anaesthesia in surprisingly meaningful ways. Understanding this link not only sharpens pharmacology knowledge but also helps you anticipate adverse reactions and counsel patients more confidently.

1. What Exactly Is PABA?

Para-aminobenzoic acid (PABA) is an aromatic compound, historically famous for two unrelated reasons:

  • It was once a major UVB filter in sunscreens.

  • It is a metabolite of ester-type local anaesthetics.

Chemically, PABA is a para-substituted benzene ring with an amine (–NH₂) and a carboxylic acid (–COOH), making it a good chromophore for UV absorption—and also a common allergen.

2. PABA in Sunscreens: Why It Fell Out of Favour

In the 1970s–1990s, PABA was widely used because it absorbs UVB radiation effectively. But it had several problems:

Why PABA sunscreens declined

  • High rates of allergy: contact dermatitis, photoallergy

  • Staining of clothing

  • Photoinstability

  • Limited UVA protection

  • Competition from better, broader-spectrum, more stable filters

Today, pure PABA is almost never used in sunscreens. Some derivatives (e.g., padimate O) remain in use but are far less common.

Clinical pearl: If a patient reports “sunscreen allergy,” ask whether they previously reacted to PABA-containing products. If so, they may also be sensitive to PABA derivatives in medications.

3. Ester Local Anaesthetics: Why PABA Appears Again

Local anaesthetics (LAs) are divided into esters and amides:

Ester-type local anaesthetics

  • Procaine

  • Tetracaine

  • Benzocaine

These all share an ester linkage. They are metabolised by plasma pseudocholinesterases, producing PABA as a metabolite.

This PABA generation has two major effects:

  1. Higher allergy risk

  2. Shorter duration of action due to rapid hydrolysis

Amide-type local anaesthetics

  • Lidocaine

  • Bupivacaine

  • Prilocaine

  • Mepivacaine

These do not produce PABA, and true allergies are exceedingly rare.

4. Allergy and Cross-Reactivity: What You Must Know

Why ester LAs have higher allergy rates

  • They produce PABA during metabolism, and PABA is a well-known allergen.

  • Reactions can be true IgE-mediated or delayed-type hypersensitivity.

Cross-reactivity patterns

  • Between esters: common (shared PABA metabolite)

  • Between amides: rare

  • Between esters and amides: essentially none

Practical clinical approach

If a patient has:

  • Allergy to an ester LA → use an amide LA.

  • Allergy to lidocaine (rare) → check whether it was due to preservatives (e.g., methylparaben), which are chemically related to PABA.

Remember: Often the preservative, not the anaesthetic, is the culprit.

5. “PABA Triangle” Mnemonic

Visualise it as a triangle, like the chemical structure of benzocaine:

Benzocain

PABA sits at the centre, and three things radiate from it:

  • Old sunscreens → contained PABA → caused allergy

  • Ester anaesthetics → metabolise to PABA → allergy risk

  • Paraben preservatives → structurally similar → allergy confusion

If you remember this triangle, you’ll never mix up the mechanisms or the allergy profiles.

6. Quick Comparison Table

Feature Ester LAs Amide LAs
Examples Procaine, benzocaine, tetracaine Lidocaine, bupivacaine, mepivacaine
Metabolism Plasma cholinesterases → PABA Hepatic metabolism
Allergy risk Higher (PABA-related) Very low
Cross-reactivity Among ester LAs Rare
Duration Shorter Longer

Conclusion: Why This Matters

Understanding PABA’s dual life in sunscreens and local anaesthetics is more than trivia. It links dermatology, pharmacology, anaesthesia, and patient counselling. More importantly, it helps you:

  • Recognise true LA allergies

  • Choose safer alternatives

  • Understand historical shifts in sunscreen chemistry

  • Communicate clearly about cross-reactivity

  • Predict adverse effects in susceptible patients

Once you know the PABA story, you’ll instantly know why benzocaine causes trouble while lidocaine usually doesn’t.