Cannabis Measurements Explained for UK Medical Patients

Understanding Cannabis Measurements

Cannabis is measured in different units depending on its form. For UK medical patients, understanding these measurements is essential for safely managing your prescription. Measurements vary between dried flower (grams), oils and liquids (milligrams), and the cannabinoid content within these products.

Flower Prescriptions: Grams Per Month

Dried cannabis flower is measured in grams. Most UK medical prescriptions range from 10 to 30 grams per month, though some patients receive higher amounts depending on their condition and tolerance. A 10-gram prescription represents roughly one week’s supply for average users, whilst 30 grams typically lasts one month with daily use. Your pharmacy will dispense flower in a container, often with a grinder included for preparation.

Oil and Liquid Dosing: Milligrams

Cannabis oils and liquids are dosed in milligrams. Standard starting doses for UK medical patients typically begin at 2.5mg or 5mg per dose. These initial low doses allow your body to adjust whilst clinicians monitor your response. Many oils come in bottles with droppers marked in millilitres, but the important measurement is the total milligrams of active cannabinoids per millilitre. A prescription might specify “5mg twice daily,” meaning you take two 5mg doses across the day.

Understanding THC Percentage

THC percentage indicates the concentration of tetrahydrocannabinol within the product. When a flower product shows “20% THC,” this means each gram contains 200 milligrams of THC. To calculate this, multiply the percentage by 1000. A 15% THC flower contains 150mg THC per gram; a 10% product contains 100mg per gram. This percentage varies between batches and strains, so always check your product label. Higher percentages don’t necessarily mean better outcomes—many medical patients benefit from lower-THC products.

CBD to THC Ratios Explained

Many medical cannabis products are prescribed as ratios, written as CBD:THC. A 1:1 ratio contains equal parts cannabidiol and tetrahydrocannabinol. A 20:1 ratio contains 20 times more CBD than THC. These ratios significantly impact effects—higher CBD ratios provide anti-inflammatory and anxiolytic benefits with minimal psychoactive effects, making them suitable for daytime use. Lower ratios (1:1 or 1:2) provide stronger symptom relief but may cause more pronounced effects. Your prescription will specify the appropriate ratio for your condition.

Calculating Your Dose from a Prescription

If prescribed “flower: 10g monthly at 15% THC, take 0.5g twice daily,” you’re receiving approximately 150mg THC daily. Calculate this by: (grams taken × THC percentage × 10) = daily THC. So 0.5g × 15% × 10 = 75mg per dose, 150mg daily. For oils, the calculation is simpler—your bottle label states milligrams per millilitre, and your prescription specifies the dose volume. Always follow your specialist’s instructions precisely.

How Long One Gram Lasts

One gram of flower typically provides 2-3 doses for most patients using inhalation methods, lasting one to two days with twice-daily use. When vaporised efficiently, one gram may stretch further. Oral consumption (infusions or oils made from flower) may provide longer-lasting effects, though absorption is slower. Individual factors including body weight, metabolism, and condition severity affect duration.

Standard Medical Doses

UK medical literature suggests starting doses of 2.5-5mg THC daily, increasing gradually every 3-5 days. Most patients stabilise between 10-20mg daily, though some require higher amounts. CBD doses range from 100mg to several hundred milligrams daily.

Converting Between Formats

Converting flower to oil equivalent: multiply grams by THC percentage, then multiply by 10. A 0.5g dose of 20% flower equals 100mg THC. Converting oils: simply read the label milligrams. Always consult your pharmacist about conversion safety when switching between formats.

Medical Disclaimer: This content is for informational purposes only and does not constitute medical advice. Medical cannabis is a prescription-only medicine in the UK. Always consult a qualified healthcare professional. CannaZen is an information platform, not a medical provider.