How to Fix a Frontal Lisp or Interdental Lisp in Speech Therapy

If your child says “thun” instead of “sun” or “thebra” instead of “zebra,” you might be hearing a frontal lisp, also known as an interdental lisp. It’s one of the most common articulation patterns SLPs treat, and thankfully, it’s also one of the most treatable.

A frontal lisp can affect a child’s speech clarity, confidence, and social comfort. The good news is that with the right strategies — and consistent practice — most children can eliminate this pattern entirely.

Let’s explore what a frontal lisp is, what causes it, and how speech-language pathologists (SLPs) help children correct it step by step.

What Is a Frontal Lisp (Interdental Lisp)?

A frontal lisp happens when the tongue pushes forward between the front teeth during sounds like /s/ and /z/. Instead of air flowing cleanly down the middle of the tongue, it escapes between the teeth — causing the sound to resemble “th.”

Examples:

• “sun” → “thun”

• “zebra” → “thebra”

• “grass” → “grath”

The term interdental lisp means “between the teeth,” describing exactly where the tongue goes.

This differs from a lateral lisp, where air escapes along the sides of the tongue and sounds “slushy.”

Why a Lisp Happens

There are several possible causes of a frontal lisp:

• Oral habits: Thumb sucking, tongue thrusting, or prolonged pacifier use can affect tongue posture.

• Developmental stage: Some toddlers naturally produce “th” instead of “s” while learning. This is normal up to about age 4½.

• Weak tongue placement awareness: The child doesn’t yet know where to position the tongue for airflow control.

• Dental or structural factors: Open bites or missing front teeth can make correct tongue placement harder.

Most children with a lisp simply need explicit instruction and consistent practice to retrain their tongue’s placement.

When to Treat a Lisp

Many children outgrow mild lisps as their oral coordination improves. However, if a child is older than 4½ or 5 and still consistently says “th” for “s” or “z,” speech therapy is recommended.

Early treatment helps prevent:

• Reinforcement of incorrect patterns

• Difficulty spelling or reading “s” and “z” words correctly

• Self-consciousness about speech as they get older

How SLPs Fix a Frontal Lisp

Speech-language pathologists follow a systematic approach that retrains tongue placement, airflow, and self-awareness. Therapy often progresses through several stages — from sound isolation to conversation.

1. Awareness and Education

First, the SLP teaches the child about where the tongue should go. Using mirrors, visuals, or even tongue diagrams, they explain how the “s” sound is made.

The child learns:

• The tongue tip stays behind the teeth, not between them.

• The air flows down the center of the tongue.

• The teeth stay close together, with a narrow opening for air to pass through.

Children often practice “snake hissing” to feel the air flowing forward.

2. Tongue Placement Practice

Once awareness improves, the SLP teaches exact tongue placement. Cues may include:

• “Keep your tongue behind your teeth like you’re saying ‘t.’”

• “Smile while you say it — it helps your tongue stay back.”

• “Bite and smile” cue — gently bite the sides of the tongue to hold it in place while practicing airflow.

Tools like mirrors, straws, or tactile cues help children see and feel what correct production looks like.

3. Sound Isolation and Airflow Control

The next step is producing the /s/ sound correctly in isolation — just the sound by itself.
Children practice long, steady airflow:

“ssssssss”

Once they can do this with accuracy and confidence, the SLP moves on to short words.

4. Sound in Syllables and Words

Practice expands to include /s/ and /z/ in different word positions:

• Initial: sun, soup, sock

• Medial: messy, bicycle, pencil

• Final: bus, grass, nose

Minimal pairs like “see” vs. “thee” help children hear the difference between correct and incorrect productions.

5. Phrases and Sentences

When word-level accuracy improves, the child practices short phrases:

• “Silly snake.”

• “Seven socks.”

• “I saw the sun.”

Sentence repetition and conversational tasks come next to strengthen carryover.

6. Generalization and Everyday Practice

Finally, the child uses clear /s/ and /z/ sounds naturally in conversation.
SLPs guide parents and teachers on how to reinforce correct speech at home and in class.

At this stage, self-monitoring is key — the child learns to catch and correct their own errors.

Helpful At-Home Practice Ideas

Consistent, playful practice makes progress stick. Parents can try these easy activities:

• Mirror play: Say “s” words together while watching in a mirror.

• Snake game: Pretend to be snakes hissing “ssss” while keeping tongues behind teeth.

• Sound sort: Find household objects with /s/ sounds (sock, spoon, soap).

• Reading time: Emphasize “s” words in books and celebrate correct productions.

• Silly sentences: Make up tongue twisters (“Seven silly seals sing songs”).

Short, positive practice sessions — 5 minutes a day — work best.

What About /z/?

Once /s/ is accurate, /z/ is usually easy to teach. The same placement applies, but the vocal cords vibrate.

SLPs often say,

“Do your /s/ sound again, but turn your voice on.”

Children feel the vibration in their throat to learn the difference between /s/ (voiceless) and /z/ (voiced).

When to Seek Professional Help

If you’ve tried modeling at home and your child still consistently lisps past age 5, or if their speech is difficult for teachers or peers to understand, it’s time for an evaluation.

SLPs can determine whether the lisp is developmental or persistent and design a personalized treatment plan.

FAQs

Is a frontal lisp normal in preschoolers?
Yes. It’s common up to around age 4½, but it should fade naturally as the child gains oral-motor control.

Can adults fix a lisp?
Absolutely. Adults can successfully eliminate lisps with targeted speech therapy and consistent practice — it’s never too late.

How long does it take to fix a lisp?
It varies. Many children make noticeable progress in 2–3 months, while others may need longer depending on frequency of practice and consistency.

Final Thoughts

A frontal lisp can be frustrating, but it’s one of the most treatable articulation patterns in speech therapy. With skilled guidance from an SLP and consistent at-home reinforcement, children can quickly replace “th” with crisp, confident “s” and “z” sounds.