S1 Wireless TENS Unit with Heat for Menstrual Cramps

iStim S1 Wireless TENS Unit with Heat for Menstrual Relief

The iStim S1 Wireless TENS Unit is a slim, rechargeable device that combines transcutaneous electrical nerve stimulation with localized heat and fits the lower abdomen or another treatment area. Its flexible wings attach directly to proprietary self-adhesive gel pads, eliminating the separate lead wires found on a conventional TENS machine.

The S1 provides a fixed 100 Hz TENS program with 15 adjustable intensity levels. Heat can be used with the electrical stimulation or turned off when TENS alone is preferred. Each treatment automatically ends after approximately 20 minutes. The device can also be used on appropriate areas of the back, waist, shoulders, arms, and legs for its other FDA-cleared pain-relief indications. 

Product highlights

• Wearable TENS and heat in one device

• Designed specifically for menstrual-cramp and general pain applications

• 15 adjustable TENS intensity levels

• Fixed 100 Hz pulse frequency

• Heat up to approximately 110°F/43°C

• TENS with heat or TENS without heat

• Flexible silicone-wing design

• No external electrode lead wires

• Rechargeable lithium-ion battery

• USB-C charging

• Automatic 20-minute shutoff

• Contact-detection safety feature

• Proprietary Japanese-gel electrode pads

• Over-the-counter Class II medical device

• Made in Taiwan

The FDA-cleared FT-615 platform is intended for temporary relief of pain associated with dysmenorrhea when used with OTC pain medication. It is also indicated for symptomatic management of chronic pain, temporary relief of sore muscles associated with exercise or normal activities, and temporary relief of minor aches through superficial heat.

iStim S1 specifications

How is the S1 different from a standard TENS unit?

A conventional TENS machine usually has a handheld controller, two lead wires, and two or four separate electrodes. The S1 integrates the controls and heating element into its flexible treatment surface.

The S1 is better suited to someone who values simplicity, warmth, and freedom from lead wires. A conventional device may be preferable for users who need several electrode-placement patterns, longer sessions, multiple TENS modes, or independent channels.

Explore more options in our TENS units collection or visit the TENS education and device guide library.

How does TENS support menstrual-cramp relief?

TENS delivers electrical pulses through electrodes attached to the skin. These pulses stimulate sensory nerves in the treatment area and may alter the way pain signals are transmitted and perceived. The sensation should feel strong and distinct but comfortable—not painful or burning.

A 2024 Cochrane review assessed TENS for primary dysmenorrhea using evidence current through April 2024. The review found that both high- and low-frequency TENS appeared to reduce period pain compared with placebo or no treatment, although effectiveness varies and TENS does not diagnose or correct an underlying gynecologic condition. 

The S1 uses a high-frequency 100 Hz output, matching the menstrual-relief pulse frequency described in its product documentation. 

What does the heat function do?

The heating surface provides localized warmth up to approximately 110°F/43°C. Heat can be used with TENS or turned off so that only electrical stimulation remains active.

Heat should feel comfortably warm. Turn the device off if it becomes excessively hot, painful, or irritating.

What is the S1 intended to relieve?

The product should not be described as treating endometriosis, fibroids, ovarian cysts, pelvic inflammatory disease, or another underlying cause of painful periods. It provides temporary symptomatic relief.

Primary vs. secondary menstrual cramps

Not all menstrual pain has the same cause.

Medical evaluation is particularly important when pain begins after years of relatively comfortable periods, becomes progressively worse, occurs outside menstruation, accompanies very heavy bleeding, causes fainting, follows a positive pregnancy test, or interferes substantially with daily life. ACOG advises that you seek care when period pain is severe, worse than usual, or repeatedly disrupts normal activities.

For related education, visit the TENS protocols and electrode-placement library.

What comes with the iStim S1?

The current iStim page lists the S1, two self-adhesive gel pads, a USB-C cable, and an instruction manual. A separate online manual summary describes two packs containing eight pad pieces, two parking boards, and a quick guide. Balego should inspect current inventory before publishing a precise pad count. 

How to use the S1

Always read the instructions supplied with the Balego product before the first treatment.

1. Charge the device

Charge the S1 for at least two hours before initial use with the supplied USB-C equipment. The available instructions caution against using a quick charger. Disconnect the device from the charger before applying it to the body. 

2. Prepare the treatment area

Apply the S1 only to normal, healthy, clean, and dry skin.

Remove:

• Lotion

• Body oil

• Sweat

• Makeup

• Topical pain cream

• Additional conductive gel

Oils and lotions can reduce electrode adhesion and produce uneven stimulation.

3. Attach the proprietary pads

Each pad has a side intended for attachment to the device and another side intended for skin contact.

1. Remove the film marked Device Side.

2. Align the exposed gel surface with the corresponding conductive area on the S1.

3. Press it down evenly.

4. Remove the skin-side protective film.

5. Retain the storage film or parking board.

Do not add electrode gel. The manufacturer specifically instructs users not to apply extra gel to the S1 pads. 

4. Position the device

Apply the complete S1 horizontally over the appropriate lower-abdominal treatment area or another region permitted by its instructions. Press both wings evenly against the skin.

The entire gel surface should remain in contact. Lifted corners can concentrate stimulation and trigger the contact-detection system.

5. Turn on the S1

Press and hold the Power/Mode button for approximately three seconds.

The heat indicator begins flashing green, and the heating function starts. If the device does not detect sufficient skin contact, it may beep and switch itself off. 

6. Activate and adjust TENS

Press the Power/Mode button briefly until the TENS indicator flashes. Use the plus and minus buttons to adjust the electrical intensity.

Begin at the lowest level and increase one step at a time. The desired sensation is strong but comfortable tingling or pulsing.

7. Turn off heat when desired

Press the Power/Mode button briefly again to turn off heat while continuing TENS. The product does not require heat to deliver electrical stimulation.

8. Complete the treatment

The S1 automatically shuts off after approximately 20 minutes. To stop early, press and hold the Power/Mode button for three seconds. 

9. Remove and store

Turn off the device before lifting it from the skin. Remove it slowly to avoid damaging the gel.

Return the pads to their storage boards and keep them in a cool, dry place.

Where should the S1 be placed for menstrual cramps?

The product is designed to conform to the lower abdominal area. Apply it only to intact skin and follow the placement illustration included with the device.

Do not place it

• Across the chest

• Over the heart

• On the front or side of the neck

• Over the head

• Directly over the mouth or throat

• Over broken, infected, inflamed, or numb skin

• Over genital tissue

• In a position where the device bridges the front and back of the torso

Some people perceive period-related discomfort primarily in the lower back. The FT-615 clearance also includes appropriate back and waist applications, but placement should still follow the enclosed instructions.

Proprietary S1 gel pads

The S1 does not accept ordinary pin-style TENS electrodes. Its pads attach directly to the conductive areas on the wearable unit.

FDA documentation warns that worn pads can create incomplete contact and higher current density in smaller areas, increasing the possibility of skin irritation.

Gel-pad care

After every treatment:

1. Turn the S1 off.

2. Remove it carefully from the skin.

3. Replace the clear film or parking board.

4. Keep the gel away from hair, lint, dust, oil, and lotion.

5. Store it in a cool, dry location.

6. Do not immerse or wash the complete device.

7. Do not use alcohol, solvent, or abrasive cleaners.

8. Do not add conductive gel or water.

Replace a pad when it is dirty, damaged, dried out, or unable to hold the entire device securely against the skin. 

Safety information

Do not use the S1 with a pacemaker, implanted defibrillator, or another implanted electronic device. The available instructions also advise against use over broken skin or while driving or operating machinery. 

Do not use the device:

• During pregnancy

• Across the chest or through the thorax

• Over the front or side of the neck

• Over an area with reduced skin sensation

• Over a suspected blood clot

• Over actively infected or inflamed skin

• In a bath, shower, or other wet environment

• While sleeping

• While connected to a charger

• At the same time as another electrical-stimulation device

• Near operating shortwave or microwave therapy equipment

Consult an appropriate healthcare professional before use when there is a heart condition, seizure disorder, impaired sensation, circulation disorder, unexplained pelvic pain, recent abdominal surgery, or uncertainty about the cause of menstrual pain.

Stop treatment if any of the following apply:

• Burning or sharp pain

• Excessive heat

• Dizziness or nausea beyond usual menstrual symptoms

• Persistent redness

• Blistering

• Skin swelling

• Unexpected bleeding

• Worsening pelvic pain

Troubleshooting guide

The available S1 instructions describe contact-detection behavior, the 20-minute timer, the charging indicator, and basic troubleshooting steps. 

Product comparison

iStim S1 vs. iStim X1 vs. Balego Digital TENS vs. Twin Stim IV

The Balego Digital Edition offers five TENS modes, adjustable pulse rate and width, dual channels, up to four separately positioned pads, and longer timer options.

Choose the S1 when:

• Menstrual-cramp relief is the principal reason for purchase.

• A slim lower-abdominal design is preferred.

• Built-in heat is important.

• External lead wires would be inconvenient.

• Simple controls are preferred over multiple programs.

Frequently asked questions

Is the iStim S1 FDA-approved?

The correct wording is FDA cleared. The underlying FT-615 Heating TENS device received 510(k) clearance under K232675 on October 5, 2023. 

Is the S1 specifically cleared for menstrual cramps?

Yes. The FDA includes temporary relief of pain associated with dysmenorrhea when used with OTC pain medication among the FT-615 indications.

Can the S1 replace pain medication?

The FDA indication describes menstrual-cramp use with over-the-counter pain medication. The page should not promise that the S1 replaces medication or medical evaluation.

Is it a TENS or EMS unit?

It is a TENS unit with heat. It is intended for sensory-level pain management, not strengthening or muscle re-education.

For information about the distinction, read Balego TENS vs. NMES.

Does the S1 have several TENS programs?

No. It uses a fixed 100 Hz TENS program with 15 intensity levels.

Does it have multiple heat levels?

The available S1 instructions describe heat as on or off and list a maximum temperature of approximately 110°F/43°C. Do not advertise several heat levels unless the manual in Balego’s current carton says otherwise.

Can TENS and heat run together?

Yes. The heat and TENS functions can operate simultaneously. Heat may also be switched off while TENS continues.

How long is a treatment?

The S1 automatically shuts off after approximately 20 minutes. 

Can the S1 be worn under clothing?

Its flexible, lead-free design is intended to be wearable. Loose clothing is preferable so it does not pull at the device or roll the pad edges. 

Can it be used on the lower back?

The FT-615 clearance includes appropriate back and waist applications. Follow the enclosed placement and safety instructions.

Can I use regular TENS electrodes?

No. The S1 uses proprietary direct-attachment 50 × 56 mm gel pads.

Can I add electrode gel?

No. Additional gel can interfere with adhesion and may damage the proprietary pads. 

Why does the device beep and shut off?

The contact sensor may not detect a complete circuit. Confirm that both pads are fully attached to clean, dry skin and replace them if they no longer adhere evenly. 

Can the S1 be used during pregnancy?

Do not apply abdominal or pelvic electrical stimulation during pregnancy. Consult an obstetric healthcare professional about appropriate pain management.

Can it be used while charging?

No. Disconnect it from the charger before applying or operating it.

Can it be used while sleeping?

No. Electrical stimulation and heated wearable devices should not be used during sleep.

Is menstrual pain always normal?

No. Severe, worsening, or newly developed period pain may be associated with an underlying condition. Seek medical evaluation when symptoms are unusual, progressively worsening, or repeatedly interfere with ordinary activities. 

Why purchase the iStim S1 from Balego?

The iStim S1 Wireless TENS Unit for Menstrual Relief combines a menstrual-specific TENS indication, built-in warmth, rechargeable operation, and a slim wearable shape without external lead wires.

Balego also provides:

• Free shipping across the United States

• A Best Price Guarantee

• Access to the complete TENS device collection

• Educational content in the TENS guide library

• Product assistance through the Balego contact page