Usually yes — ashwagandha and L-theanine are both calming supplements with no known dangerous interaction and are often combined in stress and sleep products, but because both can promote relaxation or drowsiness the main concern is additive sedation, so start low, watch how you feel, and clear it with a pharmacist first.
No authoritative source describes a dangerous interaction between ashwagandha and L-theanine, and the two are commonly co-formulated for stress and sleep. The realistic caution is overlapping calming effects: NCCIH notes that ashwagandha "seems to have sedative effects" and lists drowsiness among its side effects, and L-theanine is used to promote relaxation and can occasionally cause sleepiness — so taken together they could add up to more drowsiness than expected. Separately, ashwagandha carries its own cautions that don't go away when you add L-theanine: NCCIH reports rare cases of liver injury, says it may interact with sedatives, anti-seizure, thyroid, diabetes, blood-pressure, and immune-suppressing medicines, and only has safety data for short-term use (up to about 3 months).
Practical steps: start with the lowest dose of each, ideally try them one at a time first, and take the combination at a time when you don't need to drive or operate machinery until you know how it affects you. Watch for excessive drowsiness, dizziness, low blood pressure, or feeling overly sedated or foggy — and stop and call a clinician if you notice signs of liver trouble (nausea, loss of appetite, dark urine, yellowing of the skin or eyes, or right-upper-belly pain). Be especially careful if you also take prescription sedatives, benzodiazepines (e.g., Valium, Xanax), sleep aids, or alcohol, since ashwagandha may increase their effects. Per NCCIH, avoid ashwagandha if you are pregnant or breastfeeding, have a thyroid or autoimmune disorder, or are scheduled for surgery, and talk to your doctor first if you take medication for diabetes, blood pressure, seizures, thyroid, or immune suppression. Because both are supplements (not FDA-tested for safety the way drugs are) and dosing varies by product, confirm the specific products and doses with your pharmacist or doctor before combining.
This is general reference, not medical advice, and not a guarantee of safety. Interactions depend on your doses, health conditions, and other medicines. Always confirm with your pharmacist or doctor before combining products, and follow the dosing on each label.