Legatrin PM is a two-ingredient OTC product: quinine sulfate, a compound originally extracted from the bark of the South American cinchona tree and long used to treat muscle cramps, and diphenhydramine HCl, an antihistamine widely used as a short-term sleep aid. Together, they address both the cramp itself and the disrupted sleep that comes with it. This combination makes it unique among OTC leg cramp products.

Quinine works by reducing the excitability of muscle fibers — essentially calming down the nerve-muscle signals that trigger involuntary cramping. Diphenhydramine acts on histamine receptors in the brain to produce sedation, helping you drift off and stay asleep despite discomfort. The combination targets the two biggest problems nocturnal leg cramp sufferers face: the cramp and the sleeplessness it causes.

Available as oral caplets or tablets taken at bedtime. Not available as a liquid, powder, or sublingual form.

• Nighttime leg cramps
• Muscle spasms during sleep
• Sleep trouble due to leg pain
• Restless legs (mild, cramp-related)
• Muscle tension at night
• Occasional insomnia
• Leg discomfort in older adults
• Exercise-related nighttime cramping

• Recurrence of nighttime leg cramps with consistent short-term use

• Tonic water — Contains quinine at low concentrations (around 83 mg per liter in the US), making it the only common food/beverage source of quinine — though far below therapeutic doses used for cramps.
• Magnesium-rich foods (leafy greens, pumpkin seeds, almonds) — Magnesium plays a key role in muscle relaxation and cramp prevention — a natural complement to quinine's mechanism and one of the best dietary strategies for reducing leg cramps.
• Potassium-rich foods (bananas, sweet potatoes, avocados) — Low potassium is a well-known trigger for muscle cramping; eating potassium-rich foods supports proper electrolyte balance and may reduce cramp frequency.
• Calcium-rich foods (dairy, fortified plant milk, sardines) — Calcium is essential for proper muscle contraction and nerve signaling — diets low in calcium are associated with increased cramp susceptibility.
• Water and electrolyte drinks — Dehydration is one of the most common triggers of nocturnal leg cramps; adequate daily fluid intake is a foundational strategy for reducing their occurrence.

• Recommended daily: Standard OTC adult dose is typically 1 caplet (quinine ~200–325 mg + diphenhydramine 25–50 mg) taken at bedtime. Always follow the label on the specific product, as formulations vary.
• Best time of day: Take 30 minutes before bedtime for best results.
• With or without food: Can be taken with or without food; taking with a light snack may reduce stomach upset.
• Time to effect: Diphenhydramine begins working within 30–60 minutes for sleep; quinine's cramp-relieving effect may take several days of consistent use to show full benefit.
• How long to take it: Not recommended for long-term nightly use. Use for short periods (1–2 weeks) to address acute cramping episodes. Tolerance to diphenhydramine's sleep effect builds within days. Consult a healthcare provider if cramps persist, as an underlying cause should be investigated.